An abstract, story-driven tower defense game set in a dimension "beyond hyperspace." The basic idea is to defend your "path" by setting up various "towers," each with their own strengths and weaknesses. With 26 different enemy types, 11 tower types, and 100 levels in 6 campaigns, this game will keep you busy for a while. Fast-paced, addicting, strategic, and fun.
Be forewarned: this game takes time and dedication to appreciate. However, if you take that time, you will find in Dwarf Fortress one of the most amazing and complex simulation games that you will ever play. Combining aspects of roguelike, strategy, and "god" games, Dwarf Fortress has two modes of play: Fortress, where you oversee a dwarven civilization, and Adventure Mode, which plays like a typical roguelike game. To date I find Dwarf Fortress to be one of the most amazing games ever made, including all commercial offerings, but like I said: be warned that the game is not easily accessible! For help, look to the wiki. Also, try out Mike Mayday's Graphics Edition of the game.