While playing Egypt, a crusade was launched to one of my cities, Constantinople. And before i could gather some strong units i found myself surrounded by 4 stacks of crusading armies with nothing to fend them of except 800 militia. I also had some Trebuchet's but i never find them very useful during a city defense, so i used them for another purpose, i used them to block the road. Although i lost any way because of my serious lake of useful units and because i was completely outnumbered, still the blocking tactic helped destroying there formations and take a hole lot more enemies with me. Anyway this tactic worked out pretty well for me.

this is a screenshot from the blockade