This is the (former) church of the Pantocrator monastery, known as the Zeyrek Camii. The pantocrator is a composite of two churches and a chapel between them. It was built between 1120 and 1136. The southern church was built by Empress Eirene, wide of John II Comnenus and dedicated to St. Saviour Pantocrator, Christ Almighty.

I have always found this complex closed (and for years under reconstruction), it should be wonderful inside, with ancient floors and lots of the original marmble still in place.