NICOPOLIS AD ISTRUM. CASTELLUM AQUAE (Ivan Tsarov – tsarov@abv.bg) The remains of castellum aquae of the western aquaeduct were situated on the highest place in the western extra muros quarter of Nicopolis ad Istrum. The eastern and the western walls of castellum aquae were 12.60 m long and the northern and southern walls were 12.50 m long. The southern wall was preserved up to 5.10 m in height. Part of a barrel-vaulted ceiling covering the reservoir was documented. The faces of the wall were constructed of small ashlars of sandstone in opus incertum. The emplectum consisted of uneven stones bonded with mortar and with courses of bricks. There was an outfall for emptying the reservoir in the southern wall. A second outfall for emptying the reservoir with a canal-lock was documented in the western wall of castellum aquae. Its bottom was paved with limestone ashlars connected with clamps. There was an inscription with a red-brown paint on one ashlar that reads: ΔЄΙΑ. The ashlars were arranged over a layer of mortar which covered courses of bricks built over a foundation of uneven stones bonded with mortar. A water-conduit of terracotta-pipes, dated by three coins of AD 325 – 350, was discovered close to the eastern wall of castellum aquae. During Period I, castellum aquae was used as complex divisorium. The presence of distributors and canal-locks indicated of direct water outfall without keeping and accumulating the water. During Period II, castellum aquae was constructed to serve as a water reservoir. Its approximate inner size was 10.80 m by 8.60 m by 4.50 m and thus, its capacity was c. 400 cbm.