On the picture: The monumental entrance to the mosque, in the middle of the northern facade.
Although still similar to late-Seljuk examples, its austerity illustrates the early-Ottoman strive for grandeur and rigor. The main feature of the whole is the muqarnas vault above the door.

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: (amongst others) Wikipedia.