Concierge.com's insider take:

Probably the only Four Seasons in a prison, and one of the smallest in the chain, but there's nothing confining about this 65-room hotel very handily midway between Hagia Sophia and the Blue Mosque. In fact, the century-old neoclassical Sultanahmet Prison, built on top of a buried Byzantine palace, was infamous as the place where the government threw dissidents along with pimps and drug dealers. Chase off any memories of Brad Davis suffering in Midnight Express, however: Nowadays, the former exercise yard is a pleasant courtyard set behind a handsome ocher facade, and any sadistic guards have been supplanted by a well-trained, obliging staff. The entrance halls have soaring stone arches and grand Ottoman-revival stairways; as for the bedrooms, even the standards (on the first floor) are big, with high ceilings and huge windows. The top-of-the-line duplex suites are fabulous with their views of the Blue Mosque and/or Hagia Sophia, but booking something in the next tier or two down might get you stuck overlooking the courtyard, while some lowly "deluxe" rooms have partial views and even a balcony. Bottom line: Ask when you book. There's a fitness center but no pool—digging's restricted because of that underground palace. Its size and reputation mean this place is always booked up well in advance, but another Four Seasons came to the rescue in June 2008, occupying a restored Bosphorus palace near the Çirağan Palace.