Castelo Velho de Freixo de Numão is a prehistoric fortified settlement west of Foz Côa, with stone remains on a hill above the dry interior landscape. It is much less famous than the rock art, but it adds another layer to the area's long human occupation.
Come for archaeology and views, not for a romantic medieval castle. The site is a good second stop if you have a car and want to widen the Côa visit beyond the museum and engravings.