The bridge across the river served a critical role, and the Roman authorities built a fort to protect it. Almost nothing remains of it because the Moors captured the city in 713 and used the Roman stones as building material for the alcazaba, i.e. the citadel protecting a town. The building was completed in 855 and is the oldest in the Iberian peninsula. The alcazaba consists of a squared line of walls, with each side measuring 130 meters (427 feet) in length and 10 meters (33 feet) in height. There were also 25 quadrangular towers. Two key access gates stand out in the complex. The first gate creates a direct link between the medieval city and the Roman bridge, serving as the primary entrance route. The second gate opens into the fortified enclosure itself, and an Arabic inscription, marking the foundation of the alcazaba, is still visible on its arch.