Castillo de Guadalerzas in Los Yébenes, Toledo province, is a fortress of Islamic origin that was extensively remodelled after the Reconquista and completed in 1078 to defend the strategic gorge passage in the Montes de Toledo. The structure features a quadrangular enclosure with corner towers and a central weapons yard, dominated by an immense keep originally designed as a hospital for knights of Santiago and the Temple. After conquest, the castle passed to the Order of Calatrava, which promoted hospital functions sustained through the late medieval period by military orders using it as lodging and hospice for cavalry. Though currently included in the Red List of endangered heritage sites with scarce remains preserved beneath a hillock, the site retains archaeological significance as evidence of multi-cultural medieval fortress architecture and the charitable role of military-religious orders.
