The only remains of the mediaeval defensive walls can be found straddling the southern stretch of the river Natisone. Here, in a splendid natural setting, the walls cross the water with a segmental arch. They incorporated the nearby St Felix’s church, which kept the relics of SS Felix and Fortunatus and which gives the arch its name. The building was erected out of town in the late 4th century and demolished in the late 18th.