Complex sentence: The Moors built a citadel, al-Mudayna, around this castle, and named the area after the nearby Manzanares River, which the Muslims called (Arabic: "source of water").
Split into simpler sentences: The Moors built a citadel, al-Mudayna, around this castle. They named the area after the nearby Manzanares River, which the Muslims called (Arabic: "source of water").