I'm barely halfway through this challenge but I just wanted to say that this is one of the best I've ever seen. I've been obsessed with ciphers ever since I was nine years old and read every book on the subject I could find in the local library, and this is just the kind of thing I was looking for when I (at long last) turned to the Internet for more interesting puzzles than the boring cryptograms in the paper, which are all monoalphabetic encryptions of things that old farts said just before they dropped dead.
Anyway, my autobiography aside, I just wanted to commend the authors for a wonderful puzzle that's hopefully going to keep me occupied for a few more enjoyable evenings.
Even though they probably won't read this since this site looks kind of abandoned... a shame, really; it's a nice place.