These are some hilarious first-person shooter/tower defense games.  While the new MOBA based version is a complete pass (it is beta, it might get better, but I do not like MOBAs, so it is a pass no matter what to me) - the first two Orcs Must Die games are awesome.  Great game play, optimized graphics, incredibly good voice acting, and a number of enemies and challenges to keep you entertained for dozens of hours.  At least long enough that the cost of the game will be easily buried by the enjoyment experienced.