Sanctum (Hybrid Tower Defense FPS) is currently 50% off and having a Free Weekend.
I think it's the most fun in a co-op FPS I've ever had, which is weird because I usually don't like TD, and I spend more time in single-player than multi! If you've ever spent countless hours playing a TD game or UT Invasion RPG this will feel very familiar.
If you do decide to play, here's my advice:
-Gatling is shit. Never use them.
-Penetrator is almost always as bad a gatling.
-Either Slow or Amp should always be included in a Survival mode. Killing Floor is good for Normal play and early Survival waves.
-Freeze Gun and Sniper are a necessity; level Sniper up by at least one for the first wave of Survival if it's not runners or gliders.
-High-leveled Holo + Amp = insta-gib, and a good early choke point of just these can be better than a bunch of towers.
-Spore Pods are bitches, but high-level AA, Sniper-Rex-Holo is the best defense. Violators are good too, but slower than Sniper-Rex-Holo combo.
-Gliders can be bitches too. Best defense is Scatter Laser or Violator (or both!), they're tricky to snipe but they go down quickly.
-Lightning is most effective with Bounce (received at 3?). Try to level up your lightnings to at least level 3, preferably 5. Max is insanely powerful, but should only be done to the most effective ones or if you have an excess of cash.
-For early play, many low-level towers is better than a few high-level towers.
-Violators and Mortars are good but conditional. They pack a punch, but they're slow. Either use a lot of them or don't bring them. If you only placed a few to cover your entire map, level them up by a lot. However, many violators > many scatters. I usually bring both depending on the map, and usually end up using more violators, but it depends upon the waves.
It's a really, really great game. Hope to see you guys online!