Orcs Must Die! is a surprisingly fun game. I picked it up on Steam while it was on sale and played it intensively for days and off and on since (a couple weeks).
The premise of the game is that you're a warmage (dude) who sets up traps and/or uses your weapons and magic to defend "the Rifts" from orcs that want to get through them -- fairly standard tower defense fare, but with "traps" instead of "towers". The levels grow and become increasingly elaborate and interesting, and you're free to pursue whatever means you prefer for defending the rifts. It's a well executed and well polished addition to the genre. Certain levels have additional things you can interact with, like rolling logs which will squish everything beneath them, which was a nice added touch.
I think the characterization of the game was part of its appeal too. All the characters (all three of them?) popped out at me. The story was generally not front and center, but I found myself thoroughly enjoying it in this game, and liking the warmage when I hadn't expected to.