A windowless heated room covered by a shallow brick dome, where the bathers took a hot bath in a stone basin. The room was heated by hot air circulating in under the floor and in channels within the walls The room was hot and moist: in addition to vapour from the hot bath, the caldarium was connected by a pottery conduit to a vaulted chamber sealed against smoke and ash and located above the furnace over the metal vessels in which water was heated. Typically, the caldarium also featured a smaller basin with cold water; in this small bath, it was possibly a portable basin.