Never heard of GodTower. Might try it when I get a chance.
I took a stab at the Impossible Quizzes a while back, but I found that they aren't so much hard as just annoyingly tricky. It can (and in some cases must) be solved almost entirely using guess-and-check. Almost. Every third or fourth question demands some real innovation. I thought I had beaten the first one already, but apparently not (I think I just got sick of it). I had been to 46 (Troglogrotto

) back then, and now I am at 80 with two skips. I'll continue tomorrow.
I am quite fond of some of Mr. Logic's riddles. (He has plenty !) simplecodeworks.com, scroll down to very bottom. He actually made the original, very first internet riddle. Before notpron. That is not to say it is better. He has a much different style.