amr ibn al-a'as had visualized that the conquest of egypt would be a walkover. this expectation turned out to be wrong. even at the outposts of pelusium and bilbeis, the muslims had met stiff resistance. the siege of pelusium had lasted for two months and that of bilbeis for one month. both battles were preludes to the siege of babylon, which was a larger and more important city. here, resistance on a larger scale was expected. after the fall of bilbeis, the muslims advanced to babylon, near modern cairo. the muslims arrived at babylon some time in may 640 ad. babylon was a fortified city, and the romans had prepared it for a siege. outside the city, a ditch had been dug, and a large force was positioned in the area between the ditch and the city walls. the muslims besieged the fort of babylon some time in may 640. the fort was a massive structure 18 metres (60 ft) high with walls more than 2 metres (6 ft) thick and studded with numerous towers and bastions. a muslim force of some 4,000 men unsuccessfully attacked the roman positions. early muslim sources place the strength of the byzantine force in babylon about six times the strength of the muslim force. for the next two months, fighting remained inconclusive, with the byzantines having the upper hand by repulsing every muslim assault.