The Castle of the Bishops of Sigüenza stands at the top of the old town, exactly where a frontier fortress should be. Its origins go back to the Islamic period, but the surviving form is tied to the powerful bishops who ruled Sigüenza as both churchmen and feudal lords.
Today the castle is a parador, so the interior is not a conventional museum. Even so, the exterior walls, gate, and views over the town make the climb worthwhile. Stop here near sunset if you can; the cathedral towers and the dry hills beyond look their best in low light.