As a squad of four marines of the Interstellar Armed Forces, players must investigate a derelict colony on the planet Vereon XII that was overwhelmed by an alien infestation. As players gain experience points between missions, they unlock new weapons and items for use in their loadouts.
Similar to Killing Floor, the game was originally developed in May 2004 as a total conversion modification for Unreal Tournament 2004. A version for the Source engine was in development in 2005 before being discontinued, as the team behind the mod was later hired by Valve for work on several Source engine games. Developed as a side-project, it was released for free alongside both the source code and special SDK tools (including "TileGen", a special level creation system where builders can arrange prefabricated rooms similar to the MapMaker system from the Timesplitters series).
The game later received a fan-made update on April 20, 2017, titled Alien Swarm: Reactive Drop (or "Reactive Drop" for short). Along with new community-made co-operative campaigns, this version features Steam Workshop support, eight-player capability, player-vs-player modes, new alien types, new weapons, optional gameplay mods, and improved spectating support.
Players can choose from one of eight characters, each split into four different classes:
All-rounders with a specialty in explosives, Officers can carry a powerful pump-action shotgun (the Vindicator) with an underbarreled grenade launcher (that fires incendiary grenades). They also have an aura of leadership surrounding them (which boosts the damage dealt and the damage resistance of all nearby squadmates).
- MSgt. Joseph "Sarge" Conrad - Focused more on a supportive role, Sarge has higher bonuses on the leadership effect and his total health, as well as an additional weapon damage bonus.
- Cpl. Karl Jaeger - Focused more on a combative role, Jaeger deals higher damage with both the Vindicator and all explosive weapons and items, alongside an additional melee damage bonus.
Special Weapon Experts
Focused on providing general firepower, Special Weapons can carry up to two powerful fully-automatic weapons (the Autorifle and the Minigun) and are the only characters whose weapons can overpenetrate targets and objects.
- "Wildcat" - Focused more on general combat, Wildcat has higher bonuses on weapon damage and reloading speed, as well as an additional movement speed bonus.
- Pvt. Thomas Wolfe - Focused more on specialized combat, Wolfe deals higher damage with both the Autorifle and the Minigun and has a highter chance to overpenetrate targets, alongside an additional health bonus.
While they do not possess the most powerful firepower, Medics have the unique ability to heal their squadmates (where otherwise they would need to heal themselves using the Healing Kit item) using both area-of-effect deployables and a dedicated Medical Gun (which fires a healing beam). While they are not required for missions, they are invaluable on missions with the Parasite enemy (as they significantly help reduce the effects and duration of the "infestation" effect).
- Cpl. Eva "Faith" Jensen - Focused more on a supportive role, Faith has higher bonuses to her healing capabilities (increasing both the effectiveness and total capacity of her healing weapons/items) and infestation curing capabilities (reducing the duration of infested marines under her care), as well as an additional movement speed bonus.
- Pvt. Leon Bastille - Focused more on a combative role, Bastille has higher bonuses to weapon damage and reloading speed, as well as an additional bonus that boosts the effectiveness of adrenaline for all squadmates.
While they do not possess the most powerful firepower, Techs have enhanced radars (which have three times the scanning range of normal radars) and can improve the speed of some tech-related actions (namely welding and Sentry Gun management) of nearby squadmates. They are also the only class that can hack mission-critical switches and computer consoles (which uses a special minigame), and as such are required in some missions (which cause an instant failure if all Techs are killed prior to the completion of all hacking challenges).
- Pvt. David "Crash" Murphy - Focused more on a combative role, Crash has a higher bonus on reloading speed, as well as additional bonuses for weapon damage and the effectiveness of explosive weapons.
- "Vegas" - Focused more on a supportive role, Vegas has a higher bonus on engineering support (which speeds up welding and Sentry Gun management of all nearby teammates), as well as additional bonuses for both health and melee damage.
Weapons & Equipment
Alien Swarm features a loadout configuration system consisting of two weapon slots and one item slot per player. Most of these require the player to level up by playing the game.
- 22A3-1 Assault Rifle - Standard-issue assault rifle with underbarrel frag grenade launcher.
- M73 Twin Pistols - Dual handguns used for ammo efficiency.
- IAF Advanced Sentry Gun - Deployable automatic sentry gun with standard autocannon.
- IAF Ammo Satchel - Deployable ammo packs.
- M42 Vindicator (Officer) - Pump-action shotgun that can also fire incendiary grenades.
- S23A SynTek Autogun (Special Weapons) - Portable gatling gun with auto-aim capability.
- IAF Heal Beacon (Medic) - Creates a beacon that heals all marines inside.
- 22A7-Z Prototype Assault Rifle (Tech) - Same as the standard assault rifle, with the addition of auto-aim capabilities and a stun grenade launcher in place of a frag grenade launcher.
- IAF Medical Gun (Medic) - Unlocked at level 9. Similar to the Medigun from Team Fortress 2. Fires a healing beam at other players. The marine can also use it to heal himself.
- IAF Minigun (Special Weapons) - Unlocked at level 17. Similar to the Minigun from Team Fortress 2. Creates a wide blanket of fire. Alternate fire spins up the barrel without firing.
- Model 35 Pump-action Shotgun - Unlocked at level 3. Powerful close-range double-barreled shotgun.
- IAF Tesla Cannon - Unlocked at level 5. Electric weapon that stuns enemies before killing them, arcs to other enemies, and avoids friendly fire.
- Precision Rail Rifle - Unlocked at level 7. Railgun with high accuracy and armor-piercing rounds.
- K80 Personal Defense Weapon - Unlocked at level 11. A pair of submachine guns designed for extreme damage at the cost of high ammo usage.
- M868 Flamer Unit - Unlocked at level 13. General-purpose flamethrower. However, alternate firing of this weapon extinguishes fires.
- IAF Freeze Sentry Gun - Unlocked at level 15. Deployable automatic sentry gun that fires liquid nitrogen, temporarily freezing enemies.
- AVK-36 Marksman Rifle - Unlocked at level 19. Semi-automatic sniper rifle.
- IAF Incendiary Sentry Gun - Unlocked at level 21. Deployable automatic sentry gun with incendiary bullets and faster enemy tracking.
- Chainsaw - Unlocked at level 23. Standard chainsaw with unlimited ammo.
- IAF High Velocity Sentry Cannon - Unlocked at level 25. Deployable automatic sentry gun with slugs that can travel far and create splash damage.
- Grenade Launcher - Unlocked at level 27. Handheld grenade launcher.
- Fire Extinguisher - Found in SynTek Residential. Standard fire extinguisher used to put out fires.
- Mining Laser - Found in Timor Station. Powerful laser that cuts through rocks and biomass.
- IAF Personal Healing Kit - A small medkit developed for one-time use on the marine carrying the item.
- Hand Welder - A welder that is used to weld and unweld certain doors.
- SM75 Combat Flares - A set of flares used not only to light up areas, but provide an auto-aim bonus.
- ML30 Laser Trip Mine - A pack of throwable proximity mines that blow up when any hostile passes its laser beam.
- M478 Proximity Incendiary Mines (Officer) - Unlocked at level 16. Releases a line of fire to either side of the mine.
- 13a Tactical Heavy Armor - Unlocked at level 2. Marines carrying this item automatically receive 20% less damage.
- X33 Damage Amplifier - Unlocked at level 4. Creates a beacon that doubles damage for all marines inside.
- Hornet Barrage - Unlocked at level 6. Unleashes a barrage of homing missiles.
- Freeze Grenades - Unlocked at level 8. Grenades that temporarily freezes enemies upon impact.
- Adrenaline - Unlocked at level 10. Temporarily activates slow-motion.
- IAF Tesla Sentry Coil - Unlocked at level 12. Deploys a tesla coil that stuns enemies within a certain radius.
- v45 Electric Charged Armor - Unlocked at level 14. Electric blast that stuns attackers and removes infection of parasites from the user.
- Flashlight Attachment - Unlocked at level 18. A automatic flashlight attached to the top of the weapon.
- IAF Power Fist Attachment - Unlocked at level 20. Automatically doubles melee damage of the user.
- Hand Grenades - Unlocked at level 22. A set of highly explosive cluster grenades.
- MNV34 Nightvision Goggles - Unlocked at level 24. Rechargeable night-vision goggles.
- MTD6 Smart Bomb - Unlocked at level 26. Causes a stream of homing rockets to fire over a wide area.
- Assault Jets - Found in some levels. Allows the player to perform large boosted jumps, crushing or pushing back aliens in landing.
- Blink Pack - Found in some levels. Allows the player to teleport through enemies and certain walls.
- Drone - The most common enemy found in the game. Weak on their own, they usually attack in large numbers. Drones can also leap across gaps, sometimes past defenses.
- Ranger - Starting from Deima Surface Bridge, these acid-firing aliens will start appearing. They will fire three acid pellets at the direction of a marine, which can be rolled under.
- Shield - Starting from Deima Surface Bridge, these massive aliens will occasionally appear. They are powerful close-range attackers with a large shield in the front (its front legs) that can often block attacks.
- Buzzer - Starting from Rydberg Reactor, these small, agile aliens will become an issue. They do little damage, but can fly and mess up a player's camera when attacked.
- Boomer - Starting from Rydberg Reactor, these large tripedal aliens will begin to show up. They do not attack directly, instead inflating when they see a marine. Destroying an inflated boomer will cause explosive bombs to shoot out of its body.
- Parasite - Starting from SynTek Residental, the parasite makes itself known. They are weak, but fast, and will infest marines that they manage to jump on. They can be the most powerful enemies in the game, as countering infestation requires much healing (or the use of a electric charage armor). They are often found inside swarm eggs.
- Grub - Harmless little aliens that are found around swarm eggs. Bullets do no damage, but they can be stomped.
- Mortar - Found in Sewer Junction B5 and Timor Station, the Mortar bug is a large enemy that shoots three acid bombs (similar to the Boomer) at marines at a time.
- Harvester - Found in Timor Station, the Harvester releases small bugs that look similar to parasites. These small bugs do not infect, instead exploding for high damage.
Alien Swarm comes with one seven-map campaign, titled "Jacob's Rest", in which a marine squad of the Interstellar Armed Forces must investigate a colony on Vereon XII that is overwhelmed by aliens.
- Landing Bay - Sweep the landing bay and retrieve flight data from an escape pod.
- Cargo Elevator - Proceed through the central cargo elevator to the colony.
- Deima Surface Bridge - Locate a pipeline compromised by the swarm.
- Rydberg Reactor - Gain access to the reactor control room and bring it back online.
- SynTek Residential - Search and destroy swarm eggs and biomass. (Flamethrower required for mission)
- Sewer Junction B5 - Explore the underground sewer passages while destroying swarm eggs.
- Timor Station - Locate the nuclear warhead and detonate it at the entrance to the alien hive.
Alien Swarm SDK
The Alien Swarm SDK (also known as the Alien Swarm Authoring Tools) was released along with the original launch of the Steam version of Alien Swarm. It is a special version of the Source SDK, including a suite of tools used for developing for Alien Swarm. The SDK also includes the full source code for the game, which includes updates to the Source Engine and full Steamworks integration.
While the SDK allows the player to build maps in Valve's Hammer utility, it comes with a special experimental tile-based level generator called TileGen; which allows level designers to create level arrangements using predefined rooms (in a similar fashion to the MapMaker mode in the TimeSplitters franchise). TileGen can also use predefined "mission generation rules" to randomly generate a level. More information about TileGen can be found here.