The Château de Tours sits on the south bank of the Loire just east of the Wilson Bridge, a mostly 13th- and 15th-century complex that is the surviving fragment of a much larger medieval royal residence. Two round towers — the Tour de Guise and the Logis du Gouverneur — and a stretch of curtain wall remain. The whole sits awkwardly between the modern quay and the riverbank, and is easy to walk past.
The castle now houses temporary exhibitions, run by the city in partnership with the Jeu de Paume in Paris — usually contemporary photography or art. Worth a look both for the shows and to see the inside of the towers.