In the old part of the city, you'll find the Topkapi Palace, which was the administrative center of the Ottoman Empire for most of its existence. And perhaps one of the more well-known areas in the palace is the Harem.
Don't get too excited, it's not what people usually think. The Harem was basically the place where no men (except for the head of the household) would go. Eunuchs guarded the women (some numbering in the hundreds or thousands), but there were also wives, mothers, and children who stayed in the harem - so images of a awesome high-class brothel are usually incorrect.
It wasn't that amazing of a place from an architectural or art standpoint, but that was probably more because the place was closed off to the public and it didn't need to be. It did strike all of us though as just one big prison and a bit on the claustrophobic side - not just for the women, but even the crowned princes who were required to live in specific rooms for most of their lives. Granted, they had whatever they needed, but in the case of the prince, it was still two rooms, which they called the golden cage.