Alcazaba of Mérida

The Alcazaba is a ninth-century Muslim fortification in Mérida, Spain. Like other historical edifices in the city, it is part of the UNESCO Heritage List.Located near the Roman bridge over the Guadiana river, the Puente Romano, it was built by emir Abd ar-Rahman II of Córdoba in 835 to command the city, which had rebelled in 805. It was the first Muslim alcazaba , and includes a big squared line of walls, every side measuring 130 metres in length, 10 m of height and 2.7 m thickness, built re-using Roman walls and Roman-Visigothic edifices in granite. The walls include 25 towers with quadrangular base, which also served as counterforts. Inside is an aljibe, a rainwater tank including a cistern to collect and filter water from the river.

Wikipedia