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 blend 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 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 to the 12th century, and has been reformed since then. It has an irregular floor plan, with rounded towers protected by arrow slits and, in the centre, the narrow square keep of 6 m in width, topped by battlements.