Round Tower of Ossó de Sió Monument · Ossó de Sió, Urgell, Lleida Circular tower on a 2nd-century BC Roman base of cyclopean masonry, about 24 meters in original diameter; after Ermengol IV's conquest in 1070 the medieval tower was built on top, in the settlement of Castellnou d'Ossó. It was declared a Bien Cultural de Interés Nacional (Catalonia's top heritage-protection category) in 1985. Photo: Josep Grin (CC BY-SA 3.0) What to see and do in Ossó de Sió