The oldest building not only in Icheri Sheher, the historical part of Baku, but throughout Azerbaijan is the Mosque of Muhammad (named after the architect Muhammad Abu Bakr), also known as Sinik-kala. The age of the monument is indicated by an inscription in Arabic script on a stone slab at the entrance to the mosque, which states that the mosque was built by the ustad-rais Muhammad, the son of Abu Bekra in 471 of Hijra (1078 - 1079 AD). The mosque is a two-tier structure. Its peculiarity is that the mihrab has the shape of a half cylinder and stands for the outer line of the wall. Inside, the mosque is illuminated by sunlight penetrating through small, loophole-like windows and the dim light of several lamps. Excursions are allowed in the mosque today. But, according to Muslim laws, only for men. As in every mosque, the entrance to it leads through the vaulted arch, below human height, so that each incoming person bows his head before the Almighty.