<P> Constantinople had been in existence for 874 years at the time of the Fourth Crusade and was the largest and most sophisticated city in Christendom . Almost alone amongst major medieval urban centres, it had retained the civic structures, public baths, forums, monuments, and aqueducts of classical Rome in working form . At its height, the city held an estimated population of about half a million people behind thirteen miles of triple walls . Its planned location made Constantinople not only the capital of the surviving eastern part of the Roman Empire but also a commercial centre that dominated trade routes from the Mediterranean to the Black Sea, China, India and Persia . As a result, it was both a rival and a tempting target for the aggressive new states of the west, notably the Republic of Venice . </P> <P> One of the leaders of the Third Crusade, Holy Roman Emperor Frederick I Barbarossa, openly plotted with the Serbs, Bulgars, Byzantine traitors, and even the Muslim Seljuks against the Eastern Empire and at one point sought Papal support for a crusade against the Orthodox Byzantines . Crusaders also seized the breakaway Byzantine province of Cyprus; rather than return it to the Empire, Richard I of England sold the island to the Knights Templar . Barbarossa died on crusade, and his army quickly disintegrated, leaving the English and French, who had come by sea, to fight Saladin . In 1195 Henry VI, son and heir of Barbarossa, sought to efface this humiliation by declaring a new crusade, and in the summer of 1197 a large number of German knights and nobles, headed by two archbishops, nine bishops, and five dukes, sailed for Palestine . There they captured Sidon and Beirut, but at the news of Henry's death in Messina along the way, many of the nobles and clerics returned to Europe . Deserted by much of their leadership, the rank and file crusaders panicked before an Egyptian army and fled to their ships in Tyre . </P> <P> Also in 1195, the Byzantine Emperor Isaac II Angelos was deposed in favour of his brother by a palace coup . Ascending as Alexios III Angelos, the new emperor had his brother blinded (a traditional punishment for treason, considered more humane than execution) and exiled . Ineffectual on the battlefield, Isaac had also proven to be an incompetent ruler who had let the treasury dwindle and outsourced the navy to the Venetians . His actions in wastefully distributing military weapons and supplies as gifts to his supporters had undermined the empire's defenses . The new emperor was to prove no better . Anxious to shore up his position, Alexios bankrupted the treasury . His attempts to secure the support of semi-autonomous border commanders undermined central authority . He neglected his crucial responsibilities for defence and diplomacy . The emperor's chief admiral (his wife's brother - in - law), Michael Stryphnos, reportedly sold the fleet's equipment down to the very nails to enrich himself . </P> <P> Pope Innocent III succeeded to the papacy in January 1198, and the preaching of a new crusade became the prime goal of his pontificate, expounded in his bull Post miserabile . His call was largely ignored by the European monarchs: the Germans were struggling against Papal power, and England and France were still engaged in warfare against each other . However, due to the preaching of Fulk of Neuilly, a crusading army was finally organised at a tournament held at Écry - sur - Aisne by Count Thibaut of Champagne in 1199 . Thibaut was elected leader, but he died in 1201 and was replaced by an Italian count, Boniface of Montferrat . </P>

4. discuss the impact of the crusades upon the byzantine empire