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), but in June 2008 I was shown around by the guardian (he lives to the right of the facade) - but photography was forbidden. It was as expected, with ancient floors and lots of the original marble still in place. And indeed with a number of separate spaces woven into one. Restoration was overdue.
In 2019 I visited the restored complex, used as a mosque. The sense of being in a very old building, that I had during my 2008 visit, was almost lost. But at least the building is now open to visitors (I think some parts may be closed to normal visits).

I tried to locate the opus sectile floor, but not having checked the note in the Wikipedia ("It is now hidden under the carpet of the Mosque, but can be shown upon request" I failed, people I asked about it did not know it was even there. I hope to find someone who has the knowledge and authority to show me. I intend to return.