I found this really cool site the other day that has really neat and realistic looking computer generated models of all the prominent buildings that would have been in Constantinople as of about 1200 AD. That's about 200 years earlier than the story I just posted takes place, and in between there was at least one major sacking of the city by crusaders, so it all goes downhill from here and you should picture it a lot more decrepit. Just for a sense of scale of time here, Constantinople had already been the capitol of the Byzantine Empire for about 700 years by 1200, and from 1200 till the end in 1453 is already longer than the United States has been around!
The above picture is the forum my protagonist would have walked through directly prior to entering the cistern (which itself would have looked much as it does in the picture I posted, though without the red floodlights), and this is the structure that would have been on top of the cistern.
See also, the whole city (in 1200), and the old center more close up. Though the author(s) themselves note that even at 1200 a number of these buildings were likely in ruins.
Images are listed as "no use of any kind without express permission" ... but I asked and received permission. They are all (c) Byzantium 1200 and I strongly recommend checking out the site if you're into that kind of thing.