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Origin and history

The Castan Square is a collection of archaeological remains located in the historic centre of Besançon, in the heart of the Saint-Jean district, rue de la Convention. It is an unidentified semicircular hemicycle, probably dating from the 2nd or 3rd century, at the time of the Gallo-Roman city of Vesontio. This site is located near the Black Gate, another emblematic monument of this period. The excavations revealed major architectural elements, including eight Corinthian columns and the remains of a water distribution basin, linked to the Arcier waterworks that fed the city.

Until the French Revolution, the square housed the church of Saint John the Baptist, built next to the first Baptistery of Besançon, founded in the fourth century. This Baptistery, attributed to Lin, bishop of the city in the third century by the Tribune Onnasius, used the waters of the basin of Arcier for baptisms by immersion. The church was decommissioned in 1789 and destroyed between 1794 and 1797. The ancient remains were not rediscovered until 1870 by archaeologist Auguste Castan, then librarian of the city.

Castan's excavations brought to light key elements, such as the Corinthian columns and the remains of the Arcier waterworks, 15 kilometres long. The hemicycle, with an interior diameter of 54 meters, was originally interpreted as a Roman theatre, but current specialists believe that it was rather a podium supporting a colonnade, delineating an elevated esplanade whose exact function remains mysterious. The remains were classified as historical monuments as early as 1886, followed by the garden in 1945 and the aqueduct in 2021.

An English garden, designed by architect Alfred Ducat and landscaper Brice Michel, was built in 1870 to highlight the remains. Opened to the public in 1874, he became Auguste Castan in 1898. Rehabilitation between 2004 and 2006 restored its 19th century appearance. The square thus illustrates the superposition of the epochs, mixing ancient, medieval and modern landscaped heritage.

Arcier's aqueduct, whose sections cross the communes of Vaire, Chalèze, Montfalcon and Besançon, was listed as historical monuments in 2021. This hydraulic structure, essential for the water supply of Vesontio, bears witness to Roman engineering. The square Castan, owned by the commune, remains today a place open to the public, offering a journey through time in the heart of the capital city.

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