The second installment is what is now a tower defense/hack and slash series is coming your way shortly. That is to say, Orcs Must Die! 2 is hitting 360 and PC July 30.
Orcs 2 (which seems an inferior name to Orcs Must Die! Again!) features full-on co-op and an overhauled spellbook and upgrade system so you get more of everything and can play even closer to your exact preferred style.
Note if you own the original, you'll get some exclusive, currently not detailed bonus content in the sequel.