Here valve added list of features in the Alien Swarm game.
* Tactical, 4 player co-op action game with a top-down perspective
* Complete game code and mod tools
* Unlock persistent items by gaining levels
* Over 40 weapons and equipment with countless loadout configurations
* 4 unique classes and 8 unique characters
* Matchmaking, Steam Cloud, Steam Stats
* 64 achievements
* Tile-based map generation tool
* Powered by Source and Steam
Go to here add your wishlist and you will be notify downloading the game.If you don’t have Steam account,go to here install Steam software client and register Steam account.