The vast ancient complex was located a short distance from the short side of the Circus Maximus, where the departure cages for the chariots (carceres) were. In the 3rd century AD, on the ground floor of this complex, a mithraeum was adapted, a place of worship dedicated to the god Mithras, consisting of a series of communicating rooms, covered by barrel vaults.

In the first of these is a room interpreted as a kind of sacristy (apparitorium).