The 63-room Vault Karakoy has a great location on a quiet side street, just half a block from the busy northern end of the Galata Bridge; it opened in March 2014 in a completely renovated bank building that dates to 1863.
It's a luxurious mix of old (bank vault doors and the original room radiators) and the ultramodern, including custom-designed interiors from Sinan Kafadar, blackout curtains, HD TVs, goose-down duvet and pillows and marble bathroom with rainfall showerhead.
The understated lobby/public room area has soft lighting and muted colors, making it very relaxing. In place of a big reception desk, there's a small, 24-hour concierge-type setup; the staff was really helpful when my luggage was delayed 36 hours.
The Spasoul has a wide range of services, including a traditional Turkish hammam with treatments ranging from 20 minutes to two hours. The included breakfast buffet has Turkish, Western and other items; lunch and dinner are a la carte, at reasonable prices.
Prices are realistic, given the location and level of luxury; e.g., in late March through April, rooms with breakfast and taxes start at 215 euros (about $245) for a Classic King and go up to 863 euros (about $980) for the Penthouse Suite.
Reservations can be made online, and travel agents earn commission on all bookings.