Aqua Claudia was an ancient Roman aqueduct that was begun by Emperor Caligula in 38 AD and finished by Emperor Claudius in 52 AD.
The total length was approximately 69 kilometres, most of which was underground. The flow was about 190,000 cubic metres in 24 hours. Directly after its filtering tank, near the seventh mile of the Via Latina, it finally emerged onto arches, which increase in height as the ground falls toward the city, reaching over 30 metres.
Nero extended the aqueduct with the Arcus Neroniani to the Caelian hill and Domitian further extended it to the Palatine after which the Aqua Claudia could provide all 14 Roman districts with water. The section on the Caelian hilll was called arcus Caelimontani.
The aqueduct went through at least two major repairs. Tacitus suggests that the aqueduct was in use by AD 47. An inscription from Vespasian suggests that Aqua Claudia was used for ten years, then failed and was out of use for nine years. The first repair was done by Emperor Vespasian in 71 AD; it was repaired again in 81 AD by Emperor Titus.
Alexander Severus reinforced the arches of Nero (CIL VI.1259) where they are called arcus Caelimontani, including the line of arches across the valley between the Caelian and the Palatine.
The church of San Tommaso in Formis was later built into the side of the aqueduct.References:
The city walls of Avila were built in the 11th century to protect the citizens from the Moors. They have been well maintained throughout the centuries and are now a major tourist attraction as well as a UNESCO World Heritage Site. Visitors can walk around about half of the length of the walls.
The layout of the city is an even quadrilateral with a perimeter of 2,516 m. Its walls, which consist in part of stones already used in earlier constructions, have an average thickness of 3 m. Access to the city is afforded by nine gates of different periods; twin 20 m high towers, linked by a semi-circular arch, flank the oldest ones, Puerta de San Vicente and Puerta del Alcázar.