On the northeast side of the Palazzo Imperiale was a three-storey brick-faced cistern block, a castellum aquae, dating to the late Trajanic-Hadrianic period. The north side of the cistern block rested on the pre-Trajanic mole delineating the Claudian basin and on a Trajanic mole to the south. The cisterns block also marked the northern extent of an open space bordered on the west by the Palazzo and on the south by Building 5. The Palazzo and the cistern block were connected via a vaulted structure supported on pairs of piers and paved with herringbone (opus spicatum) flooring. Additional piers supported a water channel between the first floor cisterns and the Palazzo.