PBJW is an interesting hybrid between tower defense and more traditional strategy games.  Essentially, the game is a tug-of-war between you and another player vying for territory.  The more territory you control, the more units you can place within to overwhelm the other player.  How far you push is largely dependent on how much space you use for resource generation or defense units.  If you're looking for a multiplayer game that offers a unique spin on the tower-defense-genre, give PBJW a try.