Yaqub al-Mansur (known as Moulay Yacoub in Morocco), another Almohad Caliph, moved the capital of his empire to Rabat. He built Rabat's city walls, the Kasbah of the Udayas and began construction on what would have been the world's largest mosque. However, Yaqub died and construction stopped. The ruins of the unfinished mosque, along with the Hassan Tower, still stand today.