Trenched is the most interesting take on tower defense that I've played in awhile. This game came out of nowhere, it wasn't even on my radar, but when I tried the demo on XBLA and I was hooked.
The second I got into a "mobile trench" I knew this game was something different. The World War I/II mech setting somehow works and really drew me in as I played. I did the first couple of missions solo while I felt out the mixture of laying down defensive turrets and charging with guns blazing.
Then I discovered the best part of the game, multiplayer! All of the missions can be played with friends or with a random set of allies online. Once I played a map with allies I didn't even consider going back to solo. Not only is four more fun than one but getting to see each player's customized Trench is a treat.
There's all the customization I've come to expect from a mech game. Various chassis (basically classes) with different strengths and weaknesses along with all sorts of weapons that fit into various sized slots within said chassis. I had just as much fun in my engineering chassis throwing down turrets left and right as I did in my heavy chassis running and gunning.
With a free demo, anyone considering themselves a fan of tower defense, co-op, shooters, or mech games should spend some time with Trenched. I shelled out the $15 for the full game and after playing through to completion I don't regret it.