The Square Tower is the only vestige of the complex of the Porte de La Boisse, commanding the road to Lyon. The gate, towers and adjacent walls were demolished during the reign of Louis XVI. Originally, it was the guard house of the bourgeois militia. Built in the 18th century to serve as a prison, it is of medieval appearance with thick walls, a vaulted room, and a guard house above. A small round tower on the side of the square building contains a revolving staircase. Acquired and restored by the Comité du Vieux Montluel with the support of the municipality, this building with its beautiful stone fittings currently houses an exhibition of old cameras and cinema in the second.