I checked out Gratuitous Tank Battles as my game for today, and it’s fairly standard tower defense with a few twists, like being able to design your own units, choose both the attacking and defending sides, and manually designate targets.  It’s hard to judge just how well I was actually doing though, especially since you don’t get much warning about what kinds of units are coming, so it feels like I just have to slap down a variety of defenses and hope that’s sufficient.  Not a keeper, though I think the previous game, Gratuitous Space Battles, might fit my tastes better.