When the Fourth Crusade arrived at Constantinople on 23 June 1203, the city had a population of approximately 500,000 people, a garrison of 15,000 men (including 5,000 Varangians), and a fleet of 20 galleys. For both political and financial reasons, the permanent garrison of Constantinople had been limited to a relatively small force, made up of elite guards and other specialist units. At previous times in Byzantine history when the capital had come under direct threat, it had been possible to assemble reinforcements from frontier and provincial forces. On this occasion, the suddenness of the danger posed by the Fourth Crusade put the defenders at a serious disadvantage. The main objective of the crusaders was to place Alexios IV on the Byzantine throne so that they could receive the rich payments he had promised them. Conon of Bethune delivered this ultimatum to the Lombard envoy sent by the Emperor Alexios III Angelos, who was the pretender's uncle and had seized the throne from the pretender's father Isaac II. The citizens of Constantinople were not concerned with the cause of the deposed emperor and his exiled son; hereditary right of succession had never been adopted by the empire and a palace coup between brothers was not considered illegitimate in the way it would have been in the West. First the crusaders attacked and were repulsed from the cities of Chalcedon and Chrysopolis, suburbs of the great city. They won a cavalry skirmish in which they were outnumbered, defeating 500 Byzantines with just 80 Frankish knights.