The Aqua Alexandrina was a Roman aqueduct located in the city of Rome. The 22.4 km long aqueduct carried water from Pantano Borghese to the Baths of Alexander on the Campus Martius. It remained in use from the 3rd to the 8th century AD.
The aqueduct was constructed in AD 226 as the last of the eleven ancient aqueducts of Rome. It was built under the reign of Emperor Alexander Severus to supply his enlargement of the Thermae of Nero which were renamed Thermae Alexandrinae. The aqueduct was repaired for the first time in the era of Diocletian between the 3rd and 4th century, later between the 5th and 6th century and finally in the 8th century during the reign of Pope Adrian I.
The longest continuous above-ground stretch of the aqueduct runs through the district of Centocelle along Via dei Pioppi and Via degli Olmi. Monumental arches are looming above busy Viale Palmiro Togliatti north of Via Casilina. The road runs along the old ditch of Centocelle (Fosso di Centocelle) where the arches reached a height of 20–25 m. Formerly the crossing was an impressive feature of the Roman countryside but now it is totally surrounded by a densely built residential neighbourhood. The brick surface is very well preserved here contrary to the other sections were erosion affected it heavily.
A second longest visible stretch runs along Via dell'Acquedotto Alessandrino south of Via Casilina. The arches carried the aqueduct through a valley with the lowest point at the crossing of present-day Via Carlo Della Rocca. The ruins are surrounded by houses and a public park called Parco Giordano Sangalli. The arched stretch ends at the crossing with Via dia Torpignattara.
It is possible to follow the aqueduct from Centocelle towards Pantano Borghese through open fields and scattered farmsteads until the Grande Raccordo Anulare, the great ring road of Rome. There are significantly lower arched stretches at the crossing points of ditches and hollows for example behind the Tor Tre Teste housing estate where a public park was established around the ruins.
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The Walls of Constantinople are a series of defensive stone walls that have surrounded and protected the city of Constantinople (today Istanbul) since its founding as the new capital of the Roman Empire by Constantine the Great. With numerous additions and modifications during their history, they were the last great fortification system of antiquity, and one of the most complex and elaborate systems ever built. They were also the largest and strongest fortification in both the ancient and medieval world.
Initially built by Constantine the Great, the walls surrounded the new city on all sides, protecting it against attack from both sea and land. As the city grew, the famous double line of the Theodosian Walls was built in the 5th century. Although the other sections of the walls were less elaborate, they were, when well-manned, almost impregnable for any medieval besieger.