I'm still trying to decide whether I like Sanctum or not. Tower defense isn't a genre that I generally am very fond of, and with FPS it's a love/hate relationship (hint: I suck at them). So I went in with little in the way of expecations. Suffice say my first impressions were not positive either, and there's a problem with rendering small text that makes it hard to read.
I only went back to it because of Steam's GGP. It took me some time to get the achievement, enough to get used to it, and eventually I caught a fleeting moment of satisfaction while sniping off Tanks. So I played it more, until I got my ass handed to me several times on Arc. I was this close to giving up. What made it frustrating (and uselessly so IMO) is the game doesn't retain older checkpoints and if you mess up you're forced to start all over again. Maybe Arc made a better player out of me, because Glade seemed sooo easy, or maybe the level difficulty is just uneven, not sure.