The castle of Frías is a mixture of buildings of XII, XIII, XV & XVI centuries. Probably the most impresionant rock castle at northern Castile, it was adquired by the Crown in 1202. In 1446 king Juan II left it to Pedro Fernández de Velasco, comstable of Castile & Duke of Frías.
The river Ebro is the broadest of Spain ... but later. Here, it is still rather narrow.
There is a fortified medieval bridge just under the town.