The town of Sos del Rey Católico has been declared a Historic/Artistic site. It is one of the most interesting mediaeval urban and architectural sites in the Spanish Pyrenees.
The town of Sos is set on a rocky spur and is enclosed by Gothic walls, which in some stretches merge with the buildings themselves. Seven of the ancient gates still survive: the Fuente Alta or Reina (the most imposing, under a trapezoidal tower with battlements); Jaca and Nador, each under a defensive tower; Zaragoza, flanked by two towers; El Mudo (the only one with a pointed arch), flanked by a tower; and Unicastillo and Bueno, of which only the arches and voussoirs survive.
The castle is set on a slight promontory. It dates from the 12th century, and has been reformed since then. It has an irregular floor plan, with a rounded tower protected by arrow slits and a slender keep 6 metres square in the centre, topped by battlements.