Somebody recently turned me on to Desktop Tower Defense at Hand Drawn Games and now I’m addicted. It’s one of those “simple enough to learn but hard to master” games. It’s one of the best Flash-based games I’ve played and it allows you to submit your scores and has a team feature so I can track myself against my buddies. I’m actually playing this more than I’m playing my store-bought games right now.
DTD is a puzzle/strategy game where you add defense pieces (guns) to the board to ward off waves of attackers. You start off with a small amount of money and gain more as you kill off attackers. They come in one side of the board and try to exit the other side. Your goal is to create a maze for them to follow so they’re exposed to your guns for as long as possible. You can also spend money to upgrade your guns.