Dungeon Defenders for 3 bucks on amazon. Playing with my brothers right now. More later. Over and out.
Edit: Ok I played it and its fun. Only four player but good for some fun together on a Saturday. Will need to try playing this in solo mode. I like tower defense when it's well done. This one might make the cut. Although I'm not sure if it balances more towards tower placement strategy or twitchy combat yet.