Boring but Practical: The tar pit trap is one of the first trap types that the player unlocks. It does no damage, it cannot throw enemies, and its only role is to slow down invaders who walk across it. However, it doesn't have a recharge time, (ergo, its effect is persistent) and it causes hordes of foes who charge-in to bunch up when they reach it, so it serves as a force-multiplier for Herd Hitting Attack spells and traps. As a result, it will be used in almost every level thereafter. It's also an excellent counter against those annoying premature-trap-triggering-Kobolds.
Captain Ersatz: The Apprentice heavily resembles Ash Williams in looks, attitude and style. Hell, the sequel even gives him a boomstick!
No Celebrities Were Harmed: Also, his facial features bear a strong similitude to those of Charlie Sheen... Which might as well explain why he uses the (in)famous "Winning!" catch-phrase for time to time!
The player can do this to the enemies by means of physics manipulation, such as the wind belt, springboards, and push traps. The enemies make a costly charge through a Death Course, only to get knocked back to the start of it and be forced to run it again!
Cynical Mentor: The dead teacher will berate and belittle The Apprentice every chance he gets, despite The Apprentice being what can only be described as a walking Apocalypse.
Death Course: As a player, you'd better get good at making them or you won't survive long.
Difficulty Spike: The game will stop taking prisoners once you get to the Overpass on Nightmare difficulty.
Some might consider Nightmare by itself to be this, as you no longer get any set up time before or between waves of orcs.
DLC: Two: "Artifacts of Power" adds a host of new weapons, traps and an alternate outfit for the Apprentice. "The Lost Adventures" adds a set of extra levels, one of which is an inverted version of "The Tower" from the main campaign.
Easy-Mode Mockery: If you play on Apprentice, you can only get a maximum of two skulls instead of the five-skull potential on harder difficulties.
Old Warmage: "Now she's bound the numberless horde to her will and returns to teach the Order harsh lessons in humility and subservience. But she's in for a surprise: I'm reasonably sure my apprentice is unteachable."
In fact, whenever his character appears to be developing, it goes in the same direction on both accounts: his insight and budding wisdom both come with the admission that he's basically just delaying the inevitable conquest of Earth.
In the ending the Old Warmage muses on the Apprentice's decision to seal the Rifts, thus sacrificing the benefits of magic to save Earth, and reasons that the Apprentice was the only member of the Order willing to make that sacrifice because he was a Fearless Fool. He's uncertain whether the "Fearless" part or the "Fool" part was more important.
Friendly Fireproof: You won't trigger your own traps, and aren't affected if you're caught in the area of effect of one anyway. Even certain stage hazards (like the sequel's minecarts) will clip right through you with no effect.
Genre-Busting: As stated above, the game doesn't handle or play like a typical Tower Defense game, relying significantly more on you doing damage to the orcs than your traps. Relying on traps will quickly prove suboptimal. More often than not, traps are simply there to channel, delay, soften up, and thin out the horde for the player to dispatch more easily.
Guide Dang It: Some of the levels have multiple doors, allowing foes the come from multiple directions at once. However, they tend to stagger their entrance, first breaking down one door, then another in a different round, then eventually coming through both at the same time. Rarely does one begin the level with the resources necessary to cover more than one approach, meaning that the player must know which doors disgorge The Horde in what order. The player may have to resort to playing the first few rounds and then reverting just to figure what to cover when. The final level really takes the cake for this, with its four sets of paired doors.
Averted in the sequel, where the minimap indicates which door is about to be broken through.
Happy Dance: The better you do, the better dance you'll get!
Idiot Hero: The Apprentice is dumb as a post, relentlessly enthusiastic, and very, very good at his job. He'd be (more) insufferable if he didn't show the capacity to get worried every now and then. And, of course, it's heavily implied that his survival where seemingly all other War Mages have failed is because he is just so stupidly persistent in the face of all odds.
The Apprentice: "This flimsy waist-high wooden barricade'll stop 'em!"
Jack of All Stats: The Knowledge Weaver essentially makes you this, offering general-use upgrades for you, guardians and traps, as opposed to the trap focus of the Steel Weaver and the weapon/spell focus of the Elemental Weaver.
Last of His Kind: As you progress through the campaign, the Apprentice starts to suspect that he may be the only warmage left. The sorceress confirms this, in an attempt to demoralize him. It doesn't work.
Last Stand: The game encourages this by giving the Apprentice a sharp regeneration boost when he is near the Rift. You will live longer, but die fighting the horde there and it's all over.
Laughably Evil: The orcs, obviously. More than occasionally they come with comments that will draw a smile on the player's face. Sometimes they even scream out "MOMMY!"
Life Drain: The DLC Vampiric Gauntlets have this as their primary attack.
Mana Meter: Used to fuel your spells and some weapon attacks. Recharges by itself slowly, but can be sped up by standing near the Rift or refilled instantly from a Mana Well. The last upgrade from the "elemental" weaver allows you to regain a portion of mana from crossbow and bladestaff attacks.
Mercy Mode: Fail a normal (warmage) difficulty stage twice in a row and it will ask if you want to play it on easy (Apprentice) mode.
Mind Control: How the Sorceress is uniting and controlling the races of the orcs' world. The orcs have dialogue that indicates it, as well. As you might imagine, this doesn't bode well for her when the Apprentice shuts down all magic by closing the Rifts.
Mighty Glacier: The armored ogres are fairly slow and take a ton a punishment.
Mood Whiplash: Contrasting the cartoony graphical style and amusing one-liners is a rather grim plot where the heroic faction is slowly getting picked off and pushed back, and is on the brink of invasion by foes whose own world has gone to crap.
Our Orcs Are Different: They lean heavily into the "Tolkienian" style, being individually weaker than humanoids allied with the Player Character, come in massed hordes, possess little to no on-screen culture or particular intelligence, and directed primarily by the will of a powerful Evil Overlord.
Shield-Bearing Mook: Later levels give some of the basic orcs shields. They generally only take one hit to destroy, but prevent that first hit from being a headshot.
Shoot the Medic First: Hobgoblin shamen are rare mobs who have the ability to raise other mobs from the dead. They become especially dangerous when they raise tough mobs that were hard to take down in the first place. Killing mobs in such a way that No Body Is Left Behind prevents them from doing so.
Spike Balls of Doom: The player can use them in the swinging mace trap, where they are positioned as a weighted pendulum swinging from a ceiling mount. They are arguably one of the highest damage output traps that the player can place, but are only effective if enemies can be kept underneath their arc for long enough for it to swing by, and it can only be placed in very limited areas. It is also the most expensive single trap in the game.
The sequel (see section below) adds a physics effect to the swinging mace trap. Any non-large creature not killed outright by the swinging mace will be sent flinging in the direction of the mace's swing. This can be both good (throwing enemies off cliffs or into other traps) and bad (throwing them outside of carefully prepared trap gauntlets) so they cannot be counted on as being quite as foolproof as in the first game.
Stone Wall: The Paladins the player can summon. They do pretty modest damage, but they are very tough and attract any nearby enemies that attack players and guardians. They can be overwhelmed on the front line but serve admirably as a rear-guard in a tightly packed space. The Steel Weaver can upgrade them to regenerate health and stun enemies with their sword, turning them from rear-guards to frontline shock-troops.
Stuck Items: The crossbow is always in the first spell slot.
Shout-Out: As stated above, at the end of a wave, the Apprentice may invoke Charlie Sheen's "Winning!" line.
Smash Mook: The Ogres can take a lot of punishment, and also cause much more damage if they should reach your rift. They aren't affected by physics-based traps unless you upgrade them specifically to do so.
This Loser Is You: The game starts with The Apprentice's master dying by cracking his head open on a rift staircase. As he's dying, he reflects on his awesome Orc slaughtering accomplishments and goes on to say hope is lost now that it's up to The Apprentice, given his rather low opinion of him.
Too Dumb to Live: The traps aren't exactly hidden or anything. The orcs will blindly run into them anyway. Good for you.
Some of the comments the orcs make about the sorceress in their head suggests at least part of this is her forcing them onward.
Hilariously, the orcs often remind each to watch out for traps, and then proceed walking onto said traps themselves, along with the ones who were warned.
Vent Physics: Vent Traps launch enemies up in the air, opening them up to further attacks.
The Un Fought: You never actually fight the sorceress; it is heavily implied in the ending she dies at the hands of the orcs after losing her magical control over them. In the opening of the sequel, she runs through a reopened rift, right into the Apprentice, and teams up with him to fight the orcs.
We Need a Distraction: Orcs will stop to attack bomb barrels, Decoy traps and guardians. Paladins are especially good at this, tanking groups of Orcs while you hit them with an Area of Effect attack. Kobolds will run right past them.
Goblin Sappers are a new enemy type who carry a Big Bulky Bomb made of metal with various contraptions. This acts as a kind of EMP bomb, doing little direct damage, but temporarily disabling most kinds of traps in a radius around the Sapper when it detonates. While the Kobold Sapper makes breaches in barricades and guardians, the Goblin Sapper makes breaches in dense trap clusters.
Asteroids Monster: The elementals split when they die. To get around this, try launching them off the map or polymorphing them first!
Awesome yet Practical: The Floor Scorcher trap. It has a moderately high cost, but less so than the most expensive traps, it can burn several enemies in a line-shaped area of effect in front of itself, it can be placed just outside of a chokepoint to add extra damage inside it, its primary upgrade track directly increase its damage, it functions like a weak flip-trap when something walks over it, and it can be upgraded so it can be placed on walls with any orientation, furthering its ability to concentrate damage over a line of tiles.
Babies Ever After: The Apprentice wonders if that's what he and The Sorceress are destined for if they defeat the orcs. She is not amused.
Baleful Polymorph: The Ring of Polymorph can turn any monster into any other, or even a chicken.
Boring but Practical: The Sorceress lacks the Apprentice's tar traps, but she has exclusive control over the Acid Sprayer trap. It is unlocked for her by default, costs little, and only does modest damage. However, it can be upgraded to slow down enemies, and the long area of effect in front of it both makes it excellent for Hitting A Herd and its range and low cost allows it to supplement a wide variety of other trap designs.
She also has access to Ice Vents which fire short bursts of cold that can freeze enemies solid for a few seconds.
Charged Attack: The primary fire of the Sorceress's Scepter of Dominion can be charged up to release a single, stronger bolt, or even more so to release an explosive one. One of the possible upgrades increases the affected area.
Charm Person: The secondary fire of the above does this, fitting with the Sorceress's abilities in the last game. It's a borderline Game Breaker, as it affects all enemy types, stopping the affected enemy in their tracks, making them attack their allies, making all their allies attack them, and to add insult to injury, making the affected enemy explode on death, stunning all nearby enemies. And to go even further, the stunned enemies are generally all grouped together which sets them up for another charge attack. And the other upgrade to the Scepter increases its damage versus charmed targets.
There is one weakness to this: charmed enemies share your trap immunity, and a few of them (such as mountain trolls) are nearly unkillable by your Sceptre, upgraded or not. The real early-campaign value of this, combined with the Sceptre's perfect accuracy at range, is the ability to stop cold (while they're infighting) any small group of monsters you can see.
Collapsing Ceiling Boss: The players can become this with the Boulder Chute trap. A big net placed on the ceiling that drops a load of boulders on enemies below when the player shoots it. It has a long reset time, but is an effective Herd Hitting Attack when timed well. Several are already placed in some levels during the first act (justified by taking place in a mine) and the player can eventually unlock them as a placeable trap.
Difficulty Spike: Many of the levels in Orcs Must Die! 2 are balanced for co-op play. This means that the difficulty ramps up more quickly for single-player campaigns than it did in the original.
Discard and Draw: The Order's Weavers are no longer operating, so the player cannot benefit from them. However, many of the benefits that they offered can still be accessed, either through equipping particular trinkets or choosing certain upgrades. Mechanically, this trades off spending in-level currency for spending extra skulls and filling active slots.
Emergency Weapon: Though the dwarves primarily attack by throwing grenades at medium range, they have a hammer that they can use to defend themselves in melee combat. However, lacking the paladins' damage mitigation and the elves' long range means that they are particularly vulnerable to ranged threats, such as crossbow orcs.
Endless Game: Several story mode levels, once beaten, unlock themselves in endless mode, a game type where the hordes only continue to gather in strength and are never exhausted. The goal is to survive as many waves as possible. As Loading Screen tips suggest, it is one of the most efficient ways of grinding out skulls for upgrades.
Fusion Dance: The Water Elementals from the Fire and WaterBooster Pack invert the game's usual treatment of elementals as Asteroids Monsters by having them enter the level as small waterlings. However, a killed waterling will leave behind a puddle of bubbling magical water, which will be absorbed by other nearby water elementals. This in turn causes them to grow to a larger and more powerful form which is much harder to defeat (though the largest form drops coins when they die.)
Grenade Spam: The alternate fire for the Apprentice's new blunderbuss fires grenades, as fast as you click, as long as you have mana.
Hero with Bad Publicity: Why the Apprentice is stuck working as a miner in the sequel. He doesn't have any in-demand job skills beyond "killing Orcs with magic" and people weren't exactly happy about his decision to shut off magic in the previous game.
Metal Slime: Mr. Moneybags is an armored ogre who appears in Endless mode. He doesn't attack or decrease your score if he escapes, but he drops cash with every hit.
Money Grinding: You can get unlimited amounts of skulls from each level, but there are so many more upgrades to buy!
Money Sink: As the number of skulls you can get is now unlimited, many items cost many more skulls to unlock and fully upgrade. Unlike the first game, these upgrades cover different areas and some can be bought multiple times, in comparison to the older system of a single fixed upgrade for each item. The additional outfits are particularly big Skull Sinks.
Money Spider: The Coin Trinket gives every enemy a chance to drop a coin when they die, not just ogres, albeit smaller value coins than the one dropped by larger foes. Upgrading the trinket cause these to drop more often.
Nerf: The Swinging Mace trap from the first game has been revamped a bit. Its damage appears to be lessened slightly, and is now subject to physics, knocking the enemies it does not kill outright in the direction of its swing (and often outside its own area of effect.) Understandable, considering that how powerful it was in the first game meant that it could create a near foolproof choke point, especially when multiple ones were used together.
Not Quite Dead: The old War Mage was just knocked out in the first game and not actually dead.
Old Save Bonus: On Steam, if you own the original Orcs Must Die!, you gain access to ten levels from the original game.
Only Sane Man: The Sorceress considers herself this, but it's subverted due to the fact that she's more than a little nuts herself, what with her previous villainous attempts and her current Blood Knight demeanor.
The old Warmage is the sanest of the lot and even manages to set things right despite the...quirks of his ex-apprentices.
Order Reborn: The game concludes with the Sorceress and Apprentice working together to rebuild the Order. Ironic, since the Sorceress is the main reason the Order was reduced to one member in the first place.
Rollercoaster Mine: Not exactly Minecart Madness (which involves riding carts) but present as dynamic elements in many maps are minecarts which travel at constant speed across frictionless rails. Sometimes, they run across the floor and will bowl over orcs, and sometimes the player can hit switches to redirect them. Other times they run across suspended tracks with caustic cargo, which can be spilled onto orcs below. Notably, the tracks on the ground prevent floor traps from being placed over them, forcing players to mix up their strategy a bit to accommodate the new element.
Shoot the Medic First: In addition to the first game's hobgoblin shamen, the Family TiesBooster Pack introduces the hobgoblin healer, who heals mobs while they are still alive. They are not a big issue with lesser enemies like orcs, who can generally be damaged down to death faster than the hobgoblins can heal them, but are very dangerous when paired with stronger enemies like ogres, who's ability to take excessive punishment allows them to charge through a gauntlet of traps and come out the other side freshly healed.
This is the function of dwarf guardians, who hurl grenades at enemies. These grenades can do tremendous damage to tightly packed hordes, and are a Logical Weakness of Earth Elementals, but they detonate on a fuse, requiring the player to slow down enemies in the dwarves' range for maximum effectiveness.
Turns Red: the Yetis from Are We There Yeti?! Booster Pack. Once you bring them down to half their HP, they go berserk and start acting like gnoll hunters: running faster, leaping over barricades and trying to wreck your shit.
You Have to Burn the Web: One of the Booster Packs adds the Web Sprayer trap, which sprays sticky webs over enemies, immobilizing them. Fire will remove the restraining webs, but the Web Sprayer can be upgraded to allow the burned webs to do extra damage when that happens, turning it into an effective combo maker.