Castle and necropolis of La Peñilla Monument · Castrejón de la Peña, Montaña Palentina, Palencia Image of Castrejón de la Peña, not of the place itself. At the spot known as La Peñilla there remain the ruins of a fortress and a medieval necropolis cut into the rock, testimony to the military control of the passes between the Sierra del Brezo and the valley of the river Boedo. Photo: Valdavia (CC BY-SA 3.0) What to see and do in Castrejón de la Peña