Metal Slug is a series of games from SNK. Basically, it's a run-and-gun shmup series. Shoot everything that takes damage, rescue prisoners and collect power-ups from their underwear, attempt to stay alive.The series' name refers to the incredibly cute, heavily armed one-seater tank shown to your left. It can duck, it can jump, it's got two vulcan miniguns... and the lowest-velocity high explosive cannon the world has ever seen. If you're truly desperate you can cause it to self-destruct, ejecting you into a somersault and rushing headlong into the enemy before blowing up For Massive Damage.The game's rather threadbare story usually involves the general Morden, who is leading a rebel army to take control of the world. Over the course of the series, he makes deals with rebels, mad scientists, demons, and even aliens in his bid to change the world.The series began with the Peregrine Falcon squad, Marco and Tarma, then added the Sparrows team, Eri and Fio, who together all form the core of the series. The fourth game relegated Tarma and Eri to the sidelines, while newcomers Trevor and Nadia took their places for a while. The sixth game notably added Ralf and Clark, the Ikari Warriors who have spent the last decade or so hanging out in the King of Fighters series. Their KoF Teammate Leona Heidern is also available in Metal Slug XX as a downloadable character.An interesting feature is semi-realistic personnel damage - one hit and you die - which applies to the characters and most of the mooks as well. If you're in a vehicle, you're in luck - you get three hit points, and once you lose them, you can still eject. Did we mention that very few enemies have touch-of-death? Well, it is so - if you want to kill someone you actually need to shoot or slice 'em with a melee weapon, and the same is true for them. Don't let this fool you though: the games are mercilessly hard, but in a way that makes you keep wanting to try again. Oh, and if a tank runs you over, though, it'll still kill you - unless you have a tank of your own.Much of the game's appeal stems from its detailed, fluid graphics; it employs hand-drawn animation created by artists failing to understand the concepts of immovable chins or kneeless legs. Nuanced touches abound, such as multiple melée and idle animations for both enemies and the player characters. The artists also went to the trouble of making reloading animations, though you don't actually need to reload at any point. Even the enemy grunts are quite varied; they eat, talk amongst themselves and occasionally run screaming in terror. Of course, they also have a nasty habit of jumping on your 'slug and humping the main cannon to stop you from firing. All in all, it's a great example of how to convey a huge amount of personality without any dialog or cutscenes.Now with its own Character Sheet
This series provides examples of:
Abnormal Ammo: Seriously? Nobody finds anything abnormal about bouncing explosive blobs, robotic exploding mini-cars (THAT SMILE), Stones, a knife that creates a wave of energy, a mini thunder cloud that fires lightning bolts, a floating mini satellite, a gun that rips shit up Emperor Palpatine style with arc lightning, or a revolver that shoots spinning, exploding rifles? Nope.
Advancing Boss of Doom: 2 and X have the Aeshi Nero, a giant mecha snake trying to "devour" the pillar you're on. Later on, there were runaway trains in the subway level that you had to destroy before they pushed you to the side of the screen.
Armies Are Evil: General Morden's Rebel Army show up in just about every game, and their job is to cause chaos and destruction aspayback for the death of Morden's son. There are several other, less important armies as well, like the Arabian Infantry (found early on in Metal Slug 2 and Metal Slug X), the Ptolemaic Army (your main opponents in Metal Slug 5), and the Japanese Army (found only on a secret path in Metal Slug 3)
Attack Its Weak Point: Required with some bosses, who are vulnerable only in some areas. Including Sol De Rocca (you must shoot the shining pearl in his face) and Scyther (the skull in his chest.)
The downside to tank mode? You can only shoot forward. And it takes forever to turn around.
Also, some of the weapons like the Drop Shot and the Iron Lizard look cool and explode, but the ammunition always drops, preventing you from hitting enemies above you, and aren't much more powerful than the Enemy Chaser or Rocket Launcher.
Subverted - the only bear to be found in the entire series is on a secret alternate path in Mission 3 of Metal Slug 4 (a snowy mountain)... and he helps you, attacking the intruding Yeti-like monsters that infest this route. If you shoot him a few times he'll stop helping and leave, though.
And played straight in that if you stand in the attack hitbox of his claw swipes, you die (with the same animation as though you were knifed).
Big Bad: General Morden for most of the series. However, other villains such as Rootmars, Amadeus, the Ptolemaic Army, and the Venusian Queen serve as the Big Bad of several installments of the series.
Big Eater: Nadia is a supermodel who joined the military to burn off all the food she eats, because she can't control her appetite.
Bigfoot, Sasquatch and Yeti: Metal Slug 3 had an ice cavern in Mission 2, filled with these. They had a homing ice breath that would not kill the player (unless he was a zombie) but turn them into a snowman. If the player didn't escape from the snowman fast enough, the yeti would take out a bone club and whack the player to death.
Bilingual Bonus: The Arabic writings in the first stages of 2 and X are jokes. Literally.
Not exactly: the writings say "Indigestion Restaurent", "Corrupt Bank", and the big banners behind the boss basically say "I have diarreha , Bring me a doctor".
Bloody Murder: Turning into a zombie lets you vomit acidic blood. Huh.
Bottomless Magazines: Nobody ever needs to reload - but the heroes will reload their pistols if you stop firing after about 5 or more consecutive shots and then stop, just because. The special weapons aren't unlimited, but keep all their ammo in a single magazine.
Bowdlerise: Some US home releases of the games replace Tarma's Precision F-Strike death scream with Marco's death scream. And then there's the white blood...
Brain in a Jar: Metal Slug 3's Martian Guards are brains in jars attached to robot bodies. Metal Slug 6's Mission 3 boss is a Humongous Mecha with a brain sitting on top inside a jar to serve as the head. It also has a couple eyeballs in there.
Compilation Rerelease: Metal Slug Anthology on Playstation Two, PSP, and Wii collects the first seven games in the series (1-6 and X) Though it misses the two Neo Geo Pocket games, as well as Metal Slug Advance and Metal Slug 3D, and the games suffer from slowdown you don't get with emulating the original RO Ms with MVSPSP.
Conservationof Ninjutsu: This game practically runs on this trope. One or two people versus entire regiments of both infantry and tanks, as well as alien motherships, zombies, yetis, so on and so forth? No problem. Grab your quarters.
Cool Shades: Tarma never takes off his rather expressive sunglasses.
Cursed with Awesome: Being turned into a zombie slows you down and restricts you to a pistol... But it gives you a ridiculously powerful attack in which you vomit blood which eradicates everything in front of you. Plus, most conventional weapons won't do much to you, leaving you vulnerable only to Yetis, other zombies, and bosses.
Death Is a Slap on the Wrist: It's an arcade SHMUP, so you respawn where you died within a couple of seconds. Certain console ports, however, make you restart the level if you use a continue, which becomes a problem in Metal Slug 3's last level on the Xbox port.
The fourth game: Trevor's melee attack earns extra points but immobilizes him when he uses it. He's kicking the enemy hard enough to shred them apart. Nadia, on the other hand, whips out a taser.
The sixth game introduced differences between the characters. Marco's normal handgun does more damage. Eri gets more grenades from pick-ups and can throw them at any angle. Tarma gets more hit points when piloting a vehicle, and can lock the vehicle's gun in one position while moving. Fio receives more ammunition from special gun pick-ups. Ralf can take two hits before dying, and has a special melee move called the "Vulcan Punch." (Although he receives half the ammo and grenades from pick-ups.) Clark has a special move called the Super Argentinian Backbreaker which kills any man-sized enemy in one hit, makes him temporarily invincible and earns him a large stack of points the more he combos it.
The Sergeants are an uncommon Rebel footsoldier, and they wear yellow. They are just as weak as the normal soldiers, but they always drop a pickup when killed.
The Amadeus Infantry in Metal Slug 5 are among the most dangerous Rebel foot-soldiers, and they wear blue. Some of them are armed with the 7.62mm AR-10 Autorifle, which is one of the weapons you can use.
Ptolemaic Officers wear grey uniforms and don't cover their faces like the other soldiers. They can shoot their pistols in any direction and can take an unusual amount of damage for a human enemy, but they will run if their troops are killed.
The Ptolemaic Special Forces fight like an improved form of the standard Ptolemaic guerrilla. They tend to ambush you out of doors and elevators, and they can even throw knives.
Ptolemaic Shamans can take more than one attack and can fire green, homing projectiles.
Enemy Mine: Statistically speaking, the Rebel Army is the king of this trope. They only truly appear in the first, second, third, and sixth Metal Slugs, and out of those 4, they team up with the heroes in 3.
In Metal Slug 2 their Martian allies betray them and kidnap their leader, causing the mooks to join forces with the heroes.
In Metal Slug 3 their leader was kidnapped and replaced by a disguised Martian, and after they figure this out, they join the heroes in their Battleship Raid of the Martian's Mothership.
Played with in Metal Slug 6; they team up with the Martiansagain, but a new alien species (or evil mole people that live underground?) finds Earth half-way through the game and find it ripe for the conquering. As for the favorite food of these aliens? Mars People. So now you, Morden's army, AND the Martians all team up against these new baddies.
The second alien army is referred to as being from Venus in some supplemental material. And if The Twilight Zone taught us anything, Martians and Venusians are classic rivals in their attempts to conquer Earth ("Will the Real Alien Please Stand Up?")
Everything's Better with Monkeys- The Uzi-wielding chimpanzee partner that you could release from a box. In 4, your character could actually get turned into a similar monkey, with an increase in fire rate (as their weapon somehow turns into an Uzi too).
Evil Knockoff: The Ptolemic Army from Metal Slug 5 stole the blueprints of the titular tanks which allowed them to create their own bastardized Metal Slugs. In fact the first boss is a titanic Metal Slug dubbed Metal Rear.
Metal Slug Advance for the Gameboy Advance had Formor,a larger version(3 times) of the Metal Slug
Evil Laugh: General Morden, Allen O'Neal, Rootmars, and Amadeus.
Gatling Good: See image at top of page. Some enemy soldiers wield them, for what good it does.
Giant Enemy Crab: Metal Slug 3's first mission is full of giant crabs, with a truly enormous one serving as the boss. Take the right alternate route in the fourth level and you'll find some more.
Giant Space Flea from Nowhere: Scyther, the final boss of the fifth game. After facing hordes of terrorists, robots and cultists you're greeted by a giant demon on the top of a tower. Only thing remotely connected is a single cutscene at the very beging of the game, and that's it.
XX (The PSP version of 7) added Leona. She previously appeared in the cell phone games.
High Pressure Blood: Mooks leave a quite respectable trail of blood upon being shot or cut. Shooting down a Mook with the Shotgun results in them literally exploding into a burst of blood that is roughly twice the size of their regular sprite.
Hijacked by Ganon: Inverted in Metal Slug 4. What appears to be Allen O'Neal and General Morden are actually look-alike robots manufactured by a new enemy named Amadeus.
Hyperspace Arsenal: One miniboss from X is a superheavy assault plane that carries those huge tanks that drop themeselves on you when they run out of ammo. It carries an infinite supply.
Implacable Man: Allen O'Neal, Morden's bodyguard who soaks more damage than most tanks, and manages to come back from the dead an unnerving amount of times. In Metal Slug 2, he actually gets eaten by a killer whale which even spits out his bones afterward... and yet, in Metal Slug 3 he's there again, gatling and idiotic taunts and all, and he amazes us even more by coming back from the dead again and helping us in the endgame.
Although he took a vacation after number 3, not appearing for a few games The one in Metal Slug 4 is actually a robot, he returns in Metal Slug 7.
Improbable Weapon User: When in fat mode, your melee weapon (a combat knife or hatchet) is replaced with a really big spork. Or have a seat and whip out your plus-sized belt.
Made of Iron: In a series where most characters die from a single shot, Allen and Morden both absorb several hundred rounds of machine gun fire and/or dozens of direct hits from missiles and grenades every time they appear. Allen eventually dies only to show up alive in the next game, but Morden is just knocked into submission by it. Morden also has been known to survive a gigantic metal plate crushing him and being strapped to a spaceship as it launches into space.
The Many Deaths of You: Besides getting shot or stabbed, there are countless sprite animations for death, ranging from (while in fat mode) bursting like a balloon to getting sucked up by flying tentacle aliens to the various deaths turned into zombies or mummies! Lots of these can result in some pretty severe feelings of terror.
Marathon Level: Many final stages are quite lengthy, but 3's final stage deserves a special mention. It is literally about half the game's length, taking as long as half an hour to finish.
Mascot Mook: The hapless Rebel Grunts. Many fans were saddened by their absence in the fifth game, and they even got to be playable once.
Military Mashup Machine: Almost every boss you encounter - especially in the later games - is a great hulking mishmash of military vehicles. The classic Land Battleship (by which we mean, "A battleship with giant treads bolted on") makes an appearance in Metal Slug 2, and the very first boss in Metal Slug 3 is a giant lobster with an artillery cannon fitted on top of it.
Mind Control: In Metal Slug 3 the character that you are controlling is abducted and then turned into clones and then clone zombies. In Metal Slug 6 players have their character's counterpart become controlled by the flying parasite things.
Mook Horror Show: Human enemies panic when you respawn in front of them. Except for bosses, but everyone from the lowest infantry to blue-suited doom troops loose their shit.
More Dakka: The Metal Slug franchise is heavily based off of awesome firepower, with heavy machine guns, BIG heavy machine guns, two machine guns, and other powerful weapons. This is then taken even further by the fact that players can become fat, increasing the power of their weapons (think of a heavy machine gun that is BIG and while fat). The vehicles with a huge vulcan machine gun (or 2) are also based off of this.
Taken even further in Metal Slug 6 and 7, where playing as Fio immediately gives the player the (Big) Heavy Machine Gun. She also gets 50% more ammunition upon picking up other weapons.
The zombies in the second level are caused by an alien meteorite possessed by creepy brain-dead cyclops aliens that attack by dropping spiritual tablets on you.
Nintendo Hard: Not surprising, since arcade games need to be able to get your quarters. There is a rule among fans: "When fighting a boss, use all your grenades immediately. You'll die and get a new set in a few seconds." Metal Slug 1 was hard, yet still the easiest of the games with each additional entry generally becoming harder, and one could say the purchase of any one of the games for a home console will pay for itself in saved quarters very quickly.
Mike Uyama, speedrunning legend, explicitly states here it's harder than Contra III. And he beat that game in 15 minutes.
—>From Uyama himself:"Metal Slug 3 is longer, has more randomness, bosses don't die as quickly, and the hardest difficulty is considered impossible (or nearly impossible) to beat on one life. Whereas in Contra III, the game moves quickly, but there isn't much randomness. If that doesn't convince you, Contra III took me 10 days (this includes both practice and recording), Metal Slug 3 took me over a year."
The developers know that a very tiny fraction of clears will be proper clears done on one credit, because the ending result screen shows how many continues you used up. It's not uncommon to see, for instance, a continue count of 50 or more (in other words, over 150 lives lost) on Metal Slug 3, especially given that it's longer than the other ones.
No Swastikas: Mercilessly parodied via the enemy army, who aren't actually Nazis — even in the Japanese version — but wear Nazi uniforms, and have insignia strongly resembling poorly-censored Nazi insignia: Swastikas are plain black X-es and eagles become Phoenixes.
Ralf in Metal Slug 6 and 7 can take two hits. He's just that tough!
It's probably a reference to his King of Fighters '99 victory quote "I can survive anything - even nukes!". Which makes it sort of a gameplay-influenced Shout Out.
More like a necessary element, considering how close Ralf needs to get to tanks in order to use his anti-tank Vulcan Punch.
From game 6 onward, putting Tarma in a Vehicle doubles its defense, letting you take twice as many hits.
Interestingly enough, Metal Slug Advance gave you a lifebar (refillable through eating food), but you could invoke this trope through activation of the "Paper Thin" card. (completing a level with it gave you another card for 100% Completion.)
One-Man Army: Sometimes the Mooks actually scream and run for their lives if you approach them. They know.
This is usually after they've killed you, only for you to accept the Continue screen and come back again...
For some fun, find two soldiers chatting with each other. Kill one and watch his friend freak out and run.
Painfully Slow Projectile: The Nop 03 Sarubia (tall grey tanks) specializes in shooting slow rolling bombs to force the players to dodge and jump, making them easy targets for its allies.
People Jars: Mission 5 in Metal Slug 6 sees you rescuing Mars People from these sort of jars.
Recycled Title: The 3D Metal Slug game for the PS2 is simply titled Metal Slug.
Redshirt Army: The final mission of Metal Slug 6 starts out with you leading some of Morden's soldiers through a cave leading to the alien atronghold. They only take one hit, like you, and they have reinforcements, but after around twenty are killed, no more will appear. Challenge yourself: See if you can get to the end of that section and still have at least one guy following you.
Rewarding Vandalism: If you destroy any objects in the background, chances are it'll drop a useful item.
Interestingly enough, in one of the levels of Metal Slug 2, vandalism gets punished - if you eat too much food, you'll grow to be fat. (UH OH BIG) This means you're slower and a bigger target, so don't lose your tank. Of course, being fat also upgrades all your weapons, so...
However, in Metal Slug 3, there is no speed penalty, but no weapon upgrade.
Sequel Difficulty Spike: Metal Slug 3. Good lord, Metal Slug 3. Take how many continues you took to complete 2 and double it. That's about how much you'll be spending in 3.
Made even worse in the iPhone/Android version. Whereas in 1, 2, and X you have 20 credits, in 3, there's just five. Luckily, the iPhone versions of the games have "Mission Mode" where you can instantly go to any level you've already reached. Except that mission 3 is glitched, and won't unlock in that mode unless you reach and unlock mission 4. Meaning you HAVE to beat at least level 2 and 3 in one go.
Shock and Awe: Several enemies use electrical attacks. the Sewer Sub boss from 1 and future rebels from 7 are prominent examples-the latter going so far as to have tesla coil armed specialist infantry and electrified gauntlets replacing knives as standard issue close combat weapons..
Short Range Shotgun: Taken literally and Up to Eleven. The Shotgun is basically a directed explosion; it doesn't even hit past half a screen, but whatever it hits it virtually obliterates. The "Big" Shotgun, first seen in X, does hit half the screen.
In Metal Slug 3 level 2 the boss drops a series of large black slabs with writing on them that some might say looks like the monolith from 2001ASpaceOdyssey.
Also in Metal Slug 3 there is an enemy on level 4 that looks like the brain eating bug from Starship Troopers.
And in one of mission 4's secret paths there's enemies that look like Nausicaa's Ohmu. They are not as benevolent as in the movie.
And the meat eating plants from Metal Slug 3 look like the evil plants from The Little Shop of Horrors.
The final level of Metal Slug 3 features small alien grunts who bear a striking resemblence to the tripod aliens from the musical version of War of the Worlds. Which are themselves based on the description from the novel. (Yes, it did have a description for them. The only-showing-the-Tripods bit came from later adaptations)
This appears to be an ongoing theme with Japanese-designed Martians. Google around.
The final boss of 3 looks similar to E.T., though appearance is about where the likeness stops.
Also in Metal Slug 3's final level, the Martians have elite guards who look like Daleks of Doctor Who fame.
The submarine paths in MS 3 are most likely a reference to earlier game In The Hunt (see Spiritual Successor below), right down to the part where you have to dodge lava. Another In The Hunt reference can be found in MS 2, the China stage, where you have to avoid missiles coming up from the ground: in ITH the submarine had to endure a similar stunt. The visuals are pretty similar, too.
Also, the Slug Mariner looks suspiciously (and ironically) based on a little HELICOPTER (complete with vulcan cannon,but not placed the same way as the aforementioned Slug) from Battle Chopper, another game by Irem.
Sink the Lifeboats: If you destroy one of Morden's boats, the soldiers will bail out and the ship will break apart... but when the smoke clears you'll see the boat, now tilted upward dangerously and obviously crippled, with a lone weaponless soldier desperately trying to bail it out. Go ahead, finish the boat off and get your point bonus, you monster.
It's not like you have a choice - you have to destroy every enemy to move on during most screens, and while the boat's still floating, it counts.
Do note, enemies will file out if you take too long.
Spin to Deflect Stuff: The Venusians in Metal Slug 6 use this one quite often. The even roll on the floor and spin-jump around like a certain blue hedgehog... Oh, and they kill you if you touch them while they're spinning.
Semi-subversion; the bullets do deflect off (while not harming you, thankfully), but the Venusians will still take damage.
The trooper clones in MS 3's final level (spaceship section) deflect HMG and Vulcan bullets while jumping.
Spiritual Successor: Metal Slug shares a lot of graphical and gameplay similarities with the earlier shooters In The Hunt and Gunforce 2, both of which were made by the same team. It also shares a lot of visual design and trope usage with Undercover Cops, a Beat 'em Up also developed by said team. The Neo Geo run & gunner Cyber-Lip was also apparently done by said team, though the connections aren't as obvious.
Stock Scream: If the mooks get set on fire or blown up, they'll often let rip one of these. Also, if you die... they laugh and shrug as if to say "what was the big deal?"... and freak out when you come back thanks to the magic of extra lives.
Strong Flesh Weak Steel: The special ability of one of the Ikari Warriors in part 6 is to punch right through tank armor.
Justified in Metal Slug 3's Mission 1. The water is infested with man-eating fish that will strip a character down to their skeleton should they fall in.
Incomparably frustrating if this happens while missing the jump to the Marine Slug (and associated level path).
Also justified in Metal Slug 2/X's final mission. The water is ICY COLD and a drop in it will make your character frozen in an ice block.
And justified in Metal Slug 6's mission 1's river, the current was rapid. And mission 2 had the same flying piranhas!
Surprisingly Good English: Though made in Japan, the Metal Slug 3-D cutscenes had fully-spoken English with Japanese subtitles. The regular announcer also speaks perfect English...most of the time (ROCKET LAUWNCHER!).
Survivor Guilt: At the ending of the first game, in the London level, you find what is usually presumed the daughter of a dead enemy soldier. Jump at 2:44.
She only appears if you finish in one player mode, where the credits scroll over the destruction you left behind (graves, devastated buildings, wrecked tanks, etc). Complete a co-operative game, and all the enemy forces are seen having a party instead. The outro music also changes appropriately (or inappropriately, depending on your point of view).
Done again in Metal Slug 3D, though the first "battle" is against an actual challenging enemy.
Trouser Space: The POWs take this a step further by keeping powerups in their underwear.
Turns Red: Allen O'Neil turns red gradually as you hurt him more. He might change his pattern a little bit, but the major point to him turning red is to assure you that yes, you are in fact making progress.
Uncommon Time - No two versions of Steel Beast (The boss theme of the first Metal Slug) share a time signature (Except 2 and 3).
Unexpected Gameplay Change: Occasionally, Metal Slug games will shift from a run-and-gun to a Shoot 'em Up. Okay, so they're not all that different as genres (some people class the former under the latter, even), but the way you play changes a bit.
Ungrateful Bastard: Despite saving his ass (and organization!) in both Metal Slug 2/X and 3, Morden still tries to kill the protagonists in 6 and 7/XX. Although he is leader of world wide rebellion which player opposes.
For what it's worth, though Either Morden or Rootmars will save the player at the end of Metal Slug 6, depending on what route you took in stage 4.
Updated Rerelease: Metal Slug X was a redesign of Metal Slug 2; the primary difference is that some slowdown problems were corrected.
Aside from the upgraded game engine, which helped deal with the slowdown, X also introduced the Iron Lizard, Drop Shot, Super Grenade and Enemy Chaser weapons. It also gave us the BIG versions of the Original weapons (Heavy Machine Gun, Rocket Launcher, Shotgun, Flame Shot, and even the Laser Gun), and toned down the difficulty.
XX is an updated version of Metal Slug 7 from the DS, for the PSP.
Wake Up Call Level: The last mission of Metal Slug 3. It's just as long, if not longer than the rest of the missions combined, throwing in twist after twist at you along with very hard sequences and bosses.
Walking Tank: The Slug Gunner from 5. 3 has a Mook which is quite literally a hollowed-out tank carried by two Japanese soldiers, with a third as the commander.
Wave Motion Gun: The Lasergun from Metal Slug 2 onwards. Shown LITERALLY in-game if the player fires a Lasergun or BIG Lasergun while fat.
Weaponized Animal: The Camel Slug, Elephant Slug, Ostrich Slug and Donkey Slug, animals with a rapid-fire gun attached to them. On the Rebel Army's side, we have Huge Hermit, a huge hermit crab with a tank for its shell.
What Could Have Been: Many ideas and models were scrapped from the final product, expecially in 5. Including a giant turtle-like mecha and the native zapped by the lightning at the beginning as the final boss on a levitating, laser-spitting pillar. See this page for all the stuff that people have found from the game files.
The Worf Effect: The first game sets up Morden as an influential, well-armed, and nearly indestructible general. The second and third games allow the Mars People to humiliatingly defeat him to show how much more dangerous they are. The sixth game demonstrates the Venus People as an even greater threat by showing them obliterating Morden and the Mars People.
Zeerust: Most of the "High-Tech" devices in the series are intentionally Zeerusty, from the aforementioned land battleship, to pulp robots, to 50s B movie flying saucers. In particular, many vehicles (including aircraft) appear to be constructed of heavier metals riveted together.
Zerg Rush: There are a few sequences like this in the series, but the best one might be the ridiculous Venusian onslaught in Mission 3 of Metal Slug 6. They just keep coming in from the right and coming in from the right...
Zombie Apocalypse: The second Metal Slug 3 level pits you against zombies, apparently caused by an alien-infested meteorite. You can be turned into a zombie, which kills your movement and weapon, but gives you an obscenely powerful blood vomit attack instead of grenades and makes you immune to normal mooks' attacks. Later in the game, evil duplicates of the player get turned into zombies... and unfortunately also have the blood vomit attack.
On the bright side, if they zombify you, you can use the blood vomit on them, and if you thought grenades were good for clearing the screen... then you haven't seen the blood vomit attack.
Unfortunately, the alien zombie clones can't zombify you with their blood vomit - it just kills your character outright. Which gives them a nasty advantage over you, especially if you lose your only advantage against them (namely, small hills protecting you from the blood blasts).
Better yet, occasionally during this sequence you can find an evil, non-zombie duplicate during this section. In previous sections, they died with a single bullet, but in this section they have as much health as a zombie. This means you need to use a few grenades, several bullets, or a powerful weapon to kill them while they jump around and melee/shoot you. Meanwhile, more evil duplicate zombies are coming from the left, using their vomit attack, and you're most likely out of grenades and reduced to your pistol. Have fun!