Steam Defense is a FPS-style tower defense mod with a lot of Strategy, FPS and RPG aspects involved. The mod is set in a steampunk sci-fi environment, and it's based upon the Source engine (Half-Life 2, Orange Box). It's also made with multiplayer cooperation as it's main focus. The basic gameplay evolves around defending a certain location from incoming waves of enemies. At the beginning of a game each player chooses one of two classes, "Engineer" or "Arms", and by using the various skills provided by these classes the players must defend their base. Engineers use towers and automated defense mechanisms to fight off enemies. They have to strategically plan out where to lay down defenses, and additionally provide strategic support for the other players. Armsmen use weapons and their own physical abilities to fight off enemies. They have to be very mobile to provide support wheverer it's needed, and always be on their alert against unforseen threats.
A screenshot of the "boss" enemy type.