Fortified Review (Brucenstein)
Got this looking to get a good co-op tower defense going with a friend and, hey, it was like $2 on sale. After about a half hour it's just not scratching that itch. The comparison to Orcs Must die is an easy one to make but the difference here is that the towers/traps feel almost secondary - it's more of an arena shooter with some backup.
The towers/traps you can place in any given level are relatively few, plus they feel both slow and underpowered. Arena-wise I'm pretty disappointed in the layouts; they seem pretty cramped and there's not many strategic decisions you can make with placement; just plunk something directly in the path because there isn't much space on either side to do anything else.
Maybe it gets more engaging, or maybe it's just not for me, but it certainly seems like there games out there that do this much better.