Trenched is definitely fun!
You can play it as a FPS, Tower Defense, or a hybrid of the two. The co-op multiplayer can be really good (and required, IMHO, to get a gold medal on a couple of levels).
For me, I love tinkering around with the different Trench configurations and collecting loot. Good times...
As a side note, the writing is actually pretty clever. It's the same studio that did Psychonauts.
http://www.psychonauts.com/
