The Roman amphitheatre of Caesarodunum, the ancient name for Tours, is hidden in plain sight immediately behind the cathedral. Look for the strange curve of Rue Manceau and Rue des Ursulines, which trace the outline of the arena. The eastern half of the city's medieval Cité enclosure was actually built into the surviving walls of the amphitheatre, and stretches of those original Roman blocks are still visible in the cellars of houses along the curve.
A small interpretive plaque on Rue Manceau explains the layout. The site is one of those quiet finds where the medieval and Roman city show through the modern street plan — a few minutes spent walking the curve gives a real sense of the scale of Roman Tours.