From the view on "Arco del Quejigar", proceed La Costera trail and cross the oak grove. At sign "Otin", turn right and hike the old road 10 minutes to the abandoned hamlet of Otin. From there, cross Otin gully and hike cross country east to the rim of Mascun Canyon (spectacular viewpoint). Walk along the rim north/east on a faint path 30 minutes to the bench. Then walk along the bench to the arch view. El Castillo is located on the lower level of the cliff and is inaccessible from there. Once you have spoted the big arch, walk back the way you came 800 meters and as soon as possible walk down the broken stone slope to the lower level of the cliff. Follow on the bench a very faint path (not really easy to found) north/east to the arch. The approach of the arch required no rope or difficult down climbing. Good track!