One of the administrators, the Persian Nizam-al-Mulk, became one of the greatest statesmen of medieval Islam. For twenty years, especially during the rule of Sultan Malikshah, he was the true custodian of the Seljuk state. In addition to having administrative abilities, he was an accomplished stylist whose book on statecraft, Siyasat-Namah, is a valuable source for the political thought of the time. In it he stresses the responsibilities of the ruler: for example, if a man is killed because a bridge is in disrepair, it is the fault of the ruler, for he should make it his business to apprise himself of the smallest negligences of his underlings. Nizam-al-Mulk, furthermore, was a devout and orthodox Muslim who established a system of madrasahs or theological seminaries (called nizamiyah after the first element of his name) to provide students with free education in the religious sciences of Islam, as well as in the most advanced scientific and philosophical thought of the time. The famous theologian al-Ghazali whose greatest work, the Revival of the Sciences of Religion, was a triumph of Sunni theology taught for a time at the nizamiyah schools at Baghdad and at Nishapur. Nizam-al-Mulk was the patron of the poet and astronomer 'Umar al-Khayyam (Omar Khayyam), whose verses, as translated by Edward FitzGerald in the nineteenth century, have become as familiar to English readers as the sonnets of Shakespeare.
After the death of Malikshah in 1092, internal conflict among the young heirs led to the fragmentation of the Seljuks' central authority into smaller Seljuk states led by various members of the family, and still smaller units led by regional chieftains, no one of whom was able to unite the Muslim world as still another force appeared in the Middle East: the Crusaders.
The most imposing of the many fortresses built by the Crusaders the elegant Krak des Chevaliers in Syria (top) held out against the Muslims for over a century and a half. The Crusader castle at Sidon in Lebanon (below) was abandoned after the final defeat of the Crusader Kingdom of Jerusalem.