On the picture: One out of two outer mihrabs (prayer niche) in the north facade. Their presence is related to the porch (gallery) that had been planned, but was never built, because Mehmet I died before the work was begun.
The porch was intended to be the ‘son cemaat yeri’ of the mosque (= ‘area for those who came late’), where worshippers who reached the mosque late or at the last moment, will gather and fulfil their prayer duty. The presence of secondary mihrabs makes those worshippers feel more connected to the congregation inside.

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