The Almoravides appear to have been an exceedingly energetic and turbulent race. They were, indeed, too fond of warfare, for they were constantly fighting amongst themselves when they were not at war with the Christians. Under their dominion every ruler of a city who could raise troops called himself sovereign, and made attack upon the governor of the nearest wealthy centre. The Almoravide rule was not so just and prudent as that of the Moors who preceded them, and the people groaned under its despotism. Conquest by the Almohades came as a redemption from the tyranny of the Almoravides. In Northern Africa, the land of prophets and of new sects, Mohammed, son of Abdalla, proclaimed himself the _Mehdi_, and gained the adherence of a great horde of devotees. These Unitarians were even more fervent in piety than the Almoravides. The _Mehdi's_ general, Abdelmumen, soon became the victor of Moorish Spain. Seville was secured by the invaders in 1147, and remained under the Almohade rule till 1248. The Almohades built the great mosque, with its high minaret, part of the structure being formed of stonework of the Roman period; the Alcázar, a huge palace, which extended as far as the bank of the Guadalquivir to the Golden Tower, and many other magnificent edifices. The palace of the Moorish sovereigns at Seville was erected in the form of a triangle, with the chief gate at the Torre de la Plata (Silver Tower), which stood in the Calle de Ataranzas until 1821, when it was taken down. Trade revived in the city after its capture by the Almohades; the weavers, the metal-workers, and the builders and the decorators of houses found constant employment under the new ruler, Abu Yakub Yussuf. The Christian Spaniards saw a revival of the Mohammedan fortunes, and lamented the influx of this vigorous infidel host. Earnest prayers were addressed to the knights of the Cross in all the nations of Europe beseeching succour for the faithful in Spain. Pope Innocent III. declared a crusade, and called upon foreign Christian rulers to aid the Spaniards, with the result that a number of French and English crusaders travelled to Spain. A memorable battle was fought in the Sierra Morena, the range dividing Castile from Andalusia, and the Almohade army was almost destroyed. After this repulse the Moors never made a military demonstration of any importance in Castile, but remained in Andalusia and the southern districts. Seville and Córdova each had a different governor; the Almohade unity was ruptured, and the empire was crumbling.