This is inside the Hunat Hatun Külliyesi: the usual complex of mosque, medrese and mausoleum. It was founded in 1235-38 by the Seljuk sultan Alaeddin Kaykobad's favorite Mahperi.
On the picture: Inside the prayer hall of the mosque. It is a large rectangular room (53 x 43½ m), a basilica (or pillar hall) with eight naves and lots of pillars carrying the vaults of the roof (54 pillars, to be exact). The northeastern corner of the hall is taken by the Hunat Hâtun Mausoleum. In front of the mihrab (prayer niche) and in the middle of the prayer hall the barrel vaulting has been interrupted by domes on circular drums. The windows in the drums let additional light in. The dome in front of the mihrab seems to belong to the original building plans; the much higher skylight dome in the middle has been added later on.
Correspondent: J.M.Criel, Antwerpen
Source: ‘Tarihi Kayseri Cami ve Mescidleri’ – Doç. Ilhan Özkeçeci (1997).