Alien Swarm Reactive Drop added steam workshop support to the game. Since then, custom maps can be browsed and installed very easily.
Before workshop came around, maps where to be downloaded and installed manually.
TileGen is an experimental tile-based level generator. You can build a library of rooms and then specify rules describing how those rooms should be connected to build the level. You can quickly test your level or take the result into Hammer for more refinement and packaging into a campaign.
- Make sure you have Alien Swarm and the Alien Swarm SDK installed on Steam.
- You can find the Alien Swarm SDK in Steam under the "Library - Tools" section.
- Load up Alien Swarm.
- Open the developer console by pressing the tilde (~) key. (You may need to go into Options -> Keyboard/Mouse and tick "Enable Developer console" for this to work.)
- In the console enter: sv_cheats 1
- In the console enter: asw_tilegen
- The TileGen window should open. In the top-left you can see the currently selected theme. Below that are all the room templates belonging to that theme. The main window contains a grid showing your current map layout.