Metal Slug Advance is a 2004 spinoff of the popular run and gun series developed by Noise Factory for the Game Boy Advance. It follows the adventure of Peregrine Falcon recruits Walter Ryan and Tyra Elson as the island they've been training on is invaded by General Morden's troops.
Although obviously downscaled, it is closer to the gameplay of the main arcade games than the Mission games for the Neo-Geo Pocket were and follows the usual linear 5 Missions structure. It still keeps the life gauge for when the player is on foot, though, and introduces upgrades that can be found hidden through each stage.
This game features the following tropes:
- Bonus Boss: Allen Jr. can be fought on the bottom path of the Final Mission.
- Bonus Dungeon: Literally. The post-game Dungeon level that can be unlocked during the final stage has a maze-like structure. Collecting any of the cards hidden through the level ends it, forcing you to clear it multiple times. It has no boss fight.
- Bragging Rights Reward:
- Nearly every upgrade found during and after the Final Mission is mostly pointless since the game has no more real challenges to offer.
- Several Cards that take effort to unlock have no effect, including the final one for collecting the previous 99.
- The Cameo: The main villain of the series, Morden, only appears in portraits during the Final Mission and the Dungeon. Also, the protagonists of the other games are featured in some collectable cards.
- Collection Sidequest: The game keeps track of the cards and prisoners you collect, with some cards being upgrades or optional effects.
- Continuing Is Painful: The game gives checkpoints and infinite continues, but at the cost of all the cards and prisoners you collected through the level. Press on anyway and you'll have an even harder time from a lack of upgrades.
- Double Unlock: There are 4 cards you must obtain to actually unlock the hidden rooms in Mission 3. The Ancient Soul misleads you into thinking it will open a path on its own, but you need the Weird Ruins card as well to access it.
- Dummied Out: Underwater paths were advertised even in the box, but are scrapped in the game proper. The two areas are buried in the code with a couple of unused enemies and you can tell how they would've connected to the rest of the levels, but swimming and the Slug Marine are sadly unprogrammed.
- Extended Gameplay: There's the aforementioned Dungeon where you can find cards that unlock small hidden rooms in stages 1 and 3.
- Game-Breaking Bug: The Dungeon stage is glitched such that the Prisoner #90 located towards the goal pit might not get recorded after you rescue him no matter what.
- Goomba Stomp: One card allows you to insta-kill machines by jumping on them with the Metal Slug. Even the first boss takes severe damage from this.
- Guide Dang It!:
- The main Metal Slug installments often hide prisoners in random objects or in thin air for extra points. This game requires you to perform this Easter Egg hunt not just for 100% completion, but also to acquire useful upgrades. You need the Infra-red Vision and the X-ray Vision card to reveal both prisoners and other hidden items.
- Even after getting the "Complete!" icon on each mission, you'll likely be missing some cards that are unlocked by performing specific tasks.
- Life Meter: Like in the two Mission games, while on foot you can take a few hits before dying, depending of the damage source. Driving a Metal Slug gives you an extra health bar that follows the same rules and that can be extended by the Armor cards.
- Joke Item: The Marsnium card makes your Slug invincible... but constantly drains HP and stops the effect once it hits 1. The drain is fast enough to make the ability worthless, since gas tank items don't appear often.
- The Mole: The protagonist is betrayed by their commander in Mission 3. The guy actually was Allen Jr. all along.
- One-Hit Kill: The poison gas hazards in Mission 3 kill instantly.
- One-Hit Point Wonder: The Paper-thin card reduces your health to 1 like in the main games and you'll get another card for clearing any level that way. Good thing Mission 2 starts with a Metal Slug.
- Sequential Boss: The twin Emain Macha tanks in Mission 2. After you destroy one, a Palette Swap comes in with... the same attacks but with more agression, so this boss fight actually functions exactly the same as any other in the game despite the fancy transition between phases.
- Speedrun: To collect a set of cards that depict each of the main bosses, you must kill each one as fast as possible. You're given very little time, so specific weapons must be used. It is all doable in a first playthough of each level if you happen to already know the location of required upgrades, though.
- Spiritual Successor: Not unlike Sonic Advance to Sonic Pocket Adventure, this game keeps a few things from the Metal Slug Mission games for the Neo-Geo Pocket.
- Turns Red: Every boss trembles and starts burning midway through the fight, becoming more agressive as a result.