A mithraeum was built small by design, deliberately resembling a cave, sometimes an actual natural or artificial cavern and sometimes simply a windowless room built to suggest one, since Mithraic mythology held that Mithras slew the primordial bull inside a cave, and the meeting space itself was meant to recreate that setting rather than to hold a large public congregation. The standard interior arrangement placed two long, raised benches running the length of the room on either side of a central aisle, where initiates reclined for the cult's ritual meals, facing a niche or apse at the far end holding the mithraeum's central image, the tauroctony, a relief or freestanding sculpture showing Mithras in the act of killing the bull, typically surrounded by a fixed set of secondary figures and symbols, a dog, a snake, a scorpion and two torch bearing attendants, whose consistent recurrence across widely separated mithraea suggests a shared, standardized iconographic program understood throughout the cult regardless of location.
Surviving examples range from small, simple rooms converted from existing basement or warehouse space, as at several sites in the port city of Ostia near Rome, to more elaborately decorated mithraea such as the one beneath the Church of Santa Prisca in Rome, whose wall paintings preserve unusually detailed cult imagery, and the mithraeum at Carrawburgh along Hadrian's Wall in Britain, evidence of how far the cult traveled with the Roman military. Because membership required initiation and secrecy, mithraea were built small and inconspicuous by necessity as much as symbolic intent, easily fitting within ordinary urban buildings without announcing their presence to the surrounding city.