This is the Büyük Ağa Medresesi (also called the Kapı Ağasi Medresesi, dating from 1488, near the Kuş Köprüsü (Bird's Bridge). It had fallen to ruins but has been restored and once again houses a Koran school.

Its octagonal floor plan is very rare in Ottoman architecture. A total of 20 small domed rooms open on the arched gallery surrounding the central yard. In the south lays a large domed room, which is used as prayer hall (mescit) and classroom. The entrance is situated on the northwestern side.

Correspondent: J.M.Criel, Antwerpen.
Sources: ‘Vakıf Abideler ve eski Eserler’ - Vakıflar Genel Müdürlüğü, Ankara 1983.

I managed to overlook a Küçük Ağa Medresesi further down the street.