Castillo de Alcala la Vieja is the ruined fortress that guarded the Henares valley before modern Alcala de Henares grew around its university and old town. The remains stand outside the city on higher ground, so the visit is as much a walk through the landscape as a monument stop.
Do not expect a restored castle with rooms and exhibits. The reward is the approach, the broken walls, and the view back toward Alcala and the surrounding countryside. It works well for travellers who have already seen the Cervantes and university sights and want a more physical half-day.