At first I thought it was the Citadel from Half Life. But then I found out it's the tower for that giant eye from The Lord of the Rings.
Actually, it's the tower at Isengard, that Saruman is in control of. You'd have seen it in The Fellowship Of The Ring, when Gandalf is trapped on top of it, and in The Two Towers, when it's being attacked by the Ents, and in The Return Of The King, at the beginning when the majority of the fellowship meet up at Isengard and Sarumans orb is found, the one that allows him to communicate with Sauron.
I must say, Wow. I love it. It's big, it's brilliant, it's dark, it looks exactly like the one from the movie. Absolutely brilliant. Well done Tasgall.