The good folks over at XBLAFans have assembled a guide featuring everything you need to know about Trenched, Double Fine's excellent shooter/tower defense/World War I/mech mashup title on XBLA. From the Achievements to all of the various unlockable mech parts and soldier gear you can find in the game, this guide has you covered. They've even put some maps together on each of the game's levels, so you can figure out exactly where and when you want to put defenses down against each incoming wave.
In other words, if you've got questions about anything in the game, the answers are right there. Now, as for any possible DLC ... well, there is going to be DLC, right, Double Fine?