Alien Swarm

Alien Swarm is a free top-down shoot-em-up video game by Valve Corporation. It is a remake of the Alien Swarm mod for Unreal Tournament 2004, and it was developed by the original team, which was hired by Valve during the course of the development process.

Gameplay
Alien Swarm is a top-down shoot-em-up set at a 60 degree angle. Four players can join a single co-operative game, the aim of which is to progress through science fiction themed levels while eliminating waves of aliens. Players can choose from 40 different weapons. The game includes persistent statistics, unlockables and achievements.

Classes
The game is class-based, with players choosing from the roles of Officer, Special Weapons, Medic, and Tech. Each class has two selectable characters with a difference in abilities.

Officer
Grants a passive bonus to damage and damage resistance to nearby allies. He has access to a class-restricted shotgun with an incendiary grenade launcher and has general all-around abilities. They are also capable of throwing extra amounts of explosive ordnance, and may find class-restricted ammo caches on the field for his Vindicator shotgun. The two playable characters are Sarge and Jaeger.

Special Weapons
Brings raw damage to the table and starts out with a customized autogun with superior parameters, high magazine capacity, and auto aiming ability. The Special Weapons gains access to a powerful minigun at a high enough level, and can pick up class-restricted ammo caches on the field. The two playable characters are Wildcat and Wolfe.

Medic
Is able to equip and deploy temporary healing beacons, and at a high enough level can use a "healing gun" to selectively heal teammates and themselves. The two playable characters are Faith and Bastille.

Tech
Carries a motion sensor, can set up sentry guns faster, welds doors faster, and is the only class capable of unlocking certain door panels or completing certain objectives by means of a hacking minigame. They also have access to a prototype assault rifle with auto aiming capabilities and the ability to fire stun grenades. The two playable characters are Crash and Vegas.

Development
Alien Swarm was announced in 2005, under the title of Alien Swarm: Infested, as a Source engine sequel to the original Unreal Tournament 2004 mod. However by late 2007 the development blog had stopped updating, leaving its status uncertain. In July 2010 with the announcement of Alien Swarm it was revealed that Valve had hired the team behind Alien Swarm, who had finished the mod between working on other Valve products such as Left 4 Dead and Portal 2.

SDK
An Alien Swarm software development kit, including source code was released alongside the game. It is free to all users of Steam, rather than only to owners of existing Source games (as is the case with the 'mainline' Source SDK). This allows total conversion mods, which do not rely on content from other Valve games, to be free to all as well - a significant business decision that echoes the strategy of the Unreal Development Kit.

The SDK tools include a tile-based level designing tool called TileGen, which allows level designers to prefabricate rooms and hallways that can be either arranged in a certain way or randomly arranged based on generation rules. The tool does not replace the Valve Hammer Editor, the use of which is still required before a map can be distributed.

Plot
In Jacob's Rest, the only official campaign, a swarm of invasive aliens have taken over a colonized planet in December 2052. Marines deployed by a drop ship named "The Bloodhound" arrive to search for survivors and if need be, to destroy the colony to prevent the spread of the aliens. The task force kills a large number of not only 'normal' aliens, but alien eggs, large tumour-like growths, parasites and other creatures. They soon find out that the colonists have all been killed by the alien infestation. The marines then guide a thermonuclear bomb (originally meant for excavation purposes) through the complex and activate its timer. They return to the drop ship before the bomb detonates.