El Cau is located under one of the most emblematic streets of the city, Baixada de la Misericòrdia, a 19th century street that connects the terrace of the Circus with the one occupied by the provincial forum representation square. Both spaces today have been almost completely hidden by and covered under the medieval and early modern development of the Part Alta (Tarragona’s Old Town). Like many buildings around it, the Baixada uses several vaults of the old Circus as its foundations. It is two of these vaults which host El Cau. The vaults themselves are made of a combination of the techniques of opus  quadratum and opus caementicium. The Roman remains have been modified over the centuries in order to adapt them to the needs of each era. The Roman origin of the structure, however, can still be clearly observed. Both vaults are joined by a small passage probably constructed during the Early Modern era.