A wall covered with rays of niches, in a corner room of the building. They were used to store implements and books. The four corner rooms formed the ‘zaviye’ (dervish lodge), which was integrated in this so-called ‘multi-functional mosque’.

Regarding early-Ottoman ‘multi-functional mosques’ .
The T-form ground plan of the congregation hall of early-Ottoman mosques (the so-called ‘Bursa-type’ mosques) is derived from the late-Seljuk medrese-with-covered-courtyard, with three large iwans (half-open rooms) opening upon a central space. In the older Bursa mosques, these four spaces are still distinct from each other: their floors are not on the same level.
As the first Ottoman rulers (at least up to their conquest of Constantinople) had strong ties with various Sufi Orders, it is clear that the first Bursa-type mosques were designed as multi-functional buildings to meet the needs of these brotherhoods: a place to worship (the ‘mihrab’-room), but also to teach and debate (the ‘iwan’-rooms on the left and right) and even to stay overnight (in general two or four rooms, in the corners of the building). The central space of the prayer hall acts as a ‘public square’, connecting all areas with each other.

Correspondent: J.M.Criel, Antwerpen.
Sources: ‘Bursa – Turquie’ – booklet of the Bursa Müzeleri, 1980
& ‘Encyclopedia of the Ottoman Empire’ - Gábor Ágoston & ‎Bruce Alan Masters, 2009.