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Origin and history

The Gallo-Roman enclosure of Tours, called the "castrum enclosure", was built in the first half of the fourth century to protect the Civitas Turonorum (present-day cathedral district). Built in response to increasing insecurity in Gaul, it incorporated the Roman amphitheatre as a southern bastion, using massively re-use materials (sculpted blocks, columns, funeral steles) taken from earlier monuments. Its 1,245-metre perimeter was reinforced by about fifteen towers and several doors, two of which are still visible today.

The wall, 4 to 4.90 meters wide and about 8 meters high (excluding crenelage), was built in opus mixtum (alternation of stone and brick), a technique typical of the Lower Empire. Archaeological excavations (1974–78, 1980–1982) revealed its internal structure: a stone and mortar block core, framed by careful trimmings. The towers, mostly circular (diameter 9 to 11 meters), housed floors accessible from the round road. The enclosure was partially listed as historical monuments in 1927.

The amphitheatre, transformed into a monumental gate, played a key role in the design of the castrum, whose trapezoidal route (9 hectares) took on topographical constraints and the Loire to the north. The intramural city probably housed representatives of the administrative power in the north and religious buildings in the south (primitive cathedral, St. Martin's Basilica). After the Norman raids (IXth–Xth centuries), the enclosure was repaired under Charles le Chauve (869) and integrated into medieval fortifications until the 16th century.

The remains accessible today include portions of courtine (rue des Ursulines, quai André-Malraux), towers (tour de l'Archevêché, tour du Petit-Cupidon), and visible foundations at the Museum of Fine Arts. The south-east (Viking Garden) and north-west poternes retain their original pavements. Despite successive restorations, the enclosure remains a rare testimony of late-antical defensive urban planning in Gaul, marked by rigorous planning and systematic reuse of materials.

Archaeological studies (Galinié, Wood, Seigne) have helped to reconstruct its evolution, from its construction in the fourth century to its gradual abandonment after the Middle Ages. Excavations of "Site 3" (1974–78) revealed, among other things, the thermal baths of the redeveloped Upper Empire, while geophysical prospecting (2000s) confirmed the existence of a northern gate and a bridge over the Loire. The enclosure thus illustrates the transition between the ancient open city (Caesarodunum) and the medieval fortified city.

Its partial listing as historic monuments (1927) underscores its heritage importance. The remains, free of access, offer a unique glimpse of Gallo-Roman military architecture, where there is a mix of ancient heritage (re-employment, amphitheatre) and latent-antical innovations (cennel towers, opus mixtum).

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