The first great city to which the Crusaders came in 1089 was not Jerusalem but Constantinople, the capital of the Byzantine empire. It was the Muslim threat to the empire which had led Urban II to preach the First Crusade. Almost as much as Jerusalem itself, Byzantium was the key to the foundation, survival and ultimate eclipse of the crusading kingdom. The Byzantines had developed an ideology over seven hundred years which placed Constantinople, rather than Rome or Jerusalem, at the centre of the world. The attitudes of its rulers reflected this priority, and led to tensions with the crusaders over military and diplomatic strategy. At the same time, the riches and sophistication of the great city made a lasting impression on the crusaders, even though they found Byzantine society alien and remote. In the end, the lure of the city's wealth was irresistibly fatal to the claims of Christian unity. In April 1204, the Fourth Crusade under the Venetian doge Enrico Dandolo captured and sacked Constantinople, signalling the effective end of almost a thousand years of Byzantine dominance in the east.