(Tower) Defense Boardgame (preferably co-op)?

For a while now, I've been looking for a defense/siege type boardgame. Me and my friends are avid players of Descent (Dungeon crawl/tactics game), but are growing a bit tired of the constant "Overpowered heroes slashing their way through Overlord's dungeon" theme. Instead, we'd like to play something that feels more like the heroes are fighting for survival instead of glory/treasure/females/etc.

One of my friends came up with defense scenarios/games. Are there any good games out there that have you defensing against hordes of enemies and trying to hold ground? Could be tower defense, could be abstract, could be miniatures - anything goes, and any suggestions are welcome.

(For reference, we play Agricola, Dominion, Puerto Rico, Citadels, Descent and Roborally quite easily. Some complexity should be no problem. 6-Hour wargames, however, are out of the question. I don't know anyone with enough interest in those types of games to ever get it on the table.)