The Cathedral of Sigüenza is part church, part fortress, and it sets the tone for the whole town. Begun in the 12th century after the Christian reconquest, it keeps the heavy Romanesque massing of its earliest phase, then opens inside into Gothic chapels, Renaissance additions, and a treasury of carved stone.
Do not hurry the visit. The tomb of the Doncel is the obvious highlight, but the cloister, choir, and side chapels are just as revealing. From the square outside, the twin towers make the building look almost military, a useful reminder that this was once a frontier town.