On the picture: A detail of the decoration of the small vestibule, in front of the entrance to the prayer hall.
The small pilaster ‘carrying’ the arch is adorned with a muqarnas capital in early-Ottoman style, which combines grandeur with rigor.

Muqarnas is a stalactite-like decoration initiated in Anatolia by the Seljuk (in the 11th – 12th century). They imported it from Iran. Later on it continued to be used in the late-Seljuk architecture of the Beyliks (14th century) and by the Ottomans up to the 19th century.

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