In 1173 Saladin, the Ayyubid sultan of Egypt, sent his brother Turan-Shah on an expedition to conquer Yemen. The Ayyubids gained control of Sanaʽa in 1175 and united the various Yemeni tribal states, except for the northern mountains controlled by the Zaydi imams, into one entity. The Ayyubids switched the country's official religious allegiance to the Sunni Muslim Abbasids. During the reign of the Ayyubid ''emir'' Tughtekin ibn Ayyub, the city underwent significant improvements. These included the incorporation of the garden lands on the western bank of the Sa'ilah, known as Bustan al-Sultan, where the Ayyubids built one of their palaces. Despite Sanaʽa's strategic position, the Ayyubids chose Ta'izz as their capital while Aden was their principal income-producing city.