Metal Slug Infinity (also stylized as Metal Slug ∞) was a Tower defense, idle RPG developed and published by ekkorr and licensed by SNK for Android and iOS. The game was first released in the Philippines, New Zealand, and Singapore in March 14, 2019 before being released worldwide at April 16, 2019. The game ended its services on September 30, 2021 due to termination of license agreement.
Metal Slug Infinity is a 2D tower-defense game similar to Metal Slug Defense and Metal Slug Attack. A team of six units comes from a base, runs from left-to-right eliminating enemy units, and ultimately destroys their enemy base from the right. Should enemies bypass the player's frontlines (or vice versa), then the player's units turn around to attack the intruders. It being an Idle RPG, the player has little-to-no control over the units' actions. Instead, the player manages the game's various currencies to level up units and Slugs on the fly.
As the player advances through stages, it will become apparent that they're overwhelmed, cannot level up quickly, and slowly stops progressing. Metal Slug Infinity has the Return function, where the player uses the Time Slug to backtrack a number of stages. All units have their level reset, all Gold Mines are reclosed, and the player's gold amount disappears. On the upside, returning boosts the units' stats, increases the gold regeneration rate, and rewards Medals, Gems, Unit Piece Boxes, and Honor Coins. The commander's level is increased to the maximum stage reached divided by 100 and rounded down (e.g. stage 2780 is level 27).
The game continues on generating gold and the units fighting as the player accesses menus, goes into battles, and closes the app. The units will march offline until they hit the maximum stage the player reached. The player can hit the lock icon to active Screen Saver mode. This lets the units fight in the background at usual speeds. The player exits out by tapping on the unit five times.
Numeration for gold, power, and level amounts are represented in alphabetical order. For example, 1,000 is written as 1a rather than 1k. The next letter occurs every multiple of a thousand (e.g. 1 million = 1b, 1 billion = 1c, 1 trillion = 1d, etc.). After z is aa, ab, ac and so on.
Upon reaching stage 10,000, aside from the player now fighting Regular Army units, the player enters what is called a Magnetic Storm. The storm lowers battle speed, teleport chance, stimpack (speed increase) chance, and universal unit stats by a percentage. A new Magnetic Storm occurs every 10,000 stages, and all storms are applied together.
While most action is done automatically, the player can tap on the main battle screen to do some actions. Tapping on enemy units, obstacles, and their base inflicts slight damage to them. Activating frontal attacks are done by tapping on a Slug when it is in the middle of self-destructing. Various items and POWs are found on the battlefield and grants bonuses by tapping on them; upgrading Mummy Fio increases the chance of the item being collected without player input.
- POW: Gives the player gold and increases battle speed.
- Treasure Chest: Gives the player gold.
- Fuel Cell: Increases battle speed.
- Ammo Box: Boosts active units' attack, defense, and health by 1000% for a minute.
The game uses a multitude of currencies:
- Gold's main purpose is to level up Mines and units in the main battle arena. Gold slowly increases as time advances, but unlike all other currencies, is reset to zero when Returning. Players can increase the rate of income by opening and leveling up Gold Mines with gold. Certain Gold Mine milestones have a substantial rate increase. The player can call the Genie of Lamp to provide the combined amount of all currently leveled Gold Mines.
- Medals are used to level up the Physical, Flame, and Lightning Type Mastery of all units specific to that type. They can also be used to purchase unit parts in the various shops.
- Gems are premium currency: some amounts are rewarded through daily missions, achievements, stage milestones, and events, and of course they can be directly purchased using real money. Gems are used to enhance units, promote War Books and Artifacts, merge 5+ star Slugs, and level up commander boosts.
- Unit Pieces are parts of a unit needed to unlock that specific unit or be promoted to the next grade (star) after collecting the required amount, highlighted in blue. Left over Unit Pieces are also currencies for enhancing units and War Books. Piece Boxes provide unit pieces of a certain unit at various amounts up to 1,000 pieces.
- Mastery Points are used to upgrade a unit's skill tree and boost their stats. These are obtained through the Power Defense mode.
- Souls are used to buy Artifact Fragments, Unit Pieces, and other items at the Exchange Station. These are obtained by trading excess Unit Pieces of a 10-star unit.
- Honor Coins are rewarded through the main Player vs. Player battles, World Boss battles, or by daily missions. These are used to buy Rift Detectors at the PVP store, Artifact Fragment at the Artifact store, and enhancing War Books.
- Data Chips are gathered by defeating stages in the Slug Camp. They're used to level up Slugs. A dismantled Slug returns Chips that've been spent on it.
- Quark is used to enhance Slugs. Some amount is awarded whenever S / SR / SSR Slugs are dismantled.
- Infinity Quark allows direct purchases of SSR Slugs through the Infinity shop. It can only be obtained by dismantling SR or SSR Slugs.
- Rift Stones are used to purchase or promote Artifacts. They can be obtained by defeating Artifact stages, or through purchase via Gems.
- Artifact Fragments are used to unlock and promote Artifacts. They can be obtained by defeating Artifact stages, or through purchase via Rift Stones.
- Alliance Coins are used to make purchases in the Guild Shop. They can be obtained by completing alliance-related tasks.
- Cores are used to make purchases in the World Boss shop. They can be obtained through the related Battle mode.
- Books are used to upgrade War Books. They're earned through the Boot Camp.
- Enhancement Stones are used to make enhancements to War Books. They're occasionally available for purchase through certain shops, or rarely offered as rewards for daily tasks.
- Tickets are used to access the various submodes (each one having its own ticket), bypass wait times, and obtain Unit Pieces of a player's choice. All of them are given as rewards, although a scant few a bought in stores.
- And more...
The main bulk of Metal Slug Infinity is in its various submodes used to earn rewards and permanently level up units and Slugs.
- Commander Info: Level up elemental type masteries and increase rewards and level travel.
- Units: Earn units, enhance them, equip gear, and unlock specific stats from a skill tree.
- Slugs: Earn Slugs, level them up, merge lower tier Slugs to get better tier Slugs, and promote them.
- War Books: Increase faction and Slug stats tenfold.
- Artifacts: Increase the units' stats and versatility to itself and other factions, submodes, and the main battle mode.
- Battles: Miscellaneous battles such as PVP, world co-op, and currency collection for specific unit stat modes.
- Alliance: Cooperative guild mode.
- Events: Temporary submodes that give out more rewards than usual.
- Metal Slug Infinity's promotional material originally had the working title Metal Slug Frontier, a reference to ekkorr's earlier idle RPG game, Endless Frontier.