Sanctum Tower Defense in FPS

Hey dudes.

I found this cool game the other day. It's called Sanctum. It's a mix between first person shooter and tower defense. It's awesome, just look at the trailer.  

I don't know if anyone else here have played it but I can't beat it. The thing is I suck at building towers in a smart way..

If you haven't tried it you can get the demo from the developers site or on IndieDB, that's where I found it.