The Alcazaba is the oldest surviving Muslim fortification in Spain, built by the Umayyads in the 830s to control the Roman bridge and the river crossing. The walls are square, massive, and built largely from recycled Roman granite — you can see the Roman letters on the blocks if you look. Inside, an austere parade ground is half-occupied by a later monastery built by the Knights of Santiago.
The highlight inside the walls is the aljibe, a Moorish cistern reached down a stair half-buried below the central keep. Roman columns from a temple support the vaults, the water glimmers below, and the temperature drops by ten degrees as you descend. Climb the walls afterwards for a view straight along the Puente Romano to the Guadiana beyond — the best panorama in the city.