Castle of Onda Monument · Onda, La Plana Baixa, Castellón Onda Castle, one of the most extensive in the Valencian Community, overlooks the historic quarter of Muslim layout in this town in Castellón. The original structure was built by the Muslims in the 10th century over an earlier Roman settlement, and was buried beneath the reconstructions of later centuries. Owing to its great size it came to be known as the castle of the 300 towers, as it was said to have as many towers as the year has days. It is currently undergoing restoration and houses a museum whose highlight is an exhibition of Muslim plasterwork from a palatial house of the early 13th century. Photo: Enfo (CC BY-SA 3.0) What to see and do in Onda