On the picture: the two large domes of the prayer hall, covering the south aisle where the mihrab is (on the down part of the picture) and the adjacent central room, which is well lit by a circular skylight in its center. Prism-shaped triangles connect the circular domes with the walls of the square rooms beneath.
The painted decoration is called ‘kalem işi’ (= work of the pen/paintbrush), because it is inspired on the geometric or stylised ornamentation used by Ottoman calligraphers to illuminate books. These (recently refreshed) murals are a 19th century attempt to imitate the medieval Ottoman decoration.
Between both domes: the impressive marble arch that separates them.

Correspondent: J.M.Criel, Antwerpen.
Source: ‘Vakıf Abideler ve eski Eserler (volume III)’ - Vakıflar Genel Müdürlüğü, Ankara 1983