this works like the same way in crusader, but if you select a group of units to go to a tower, and then (before the first group reaches the tower) tell another group to go there, you can pack a ton of people in a tower at once. this has its advantages and disadvantages. one, you have an extremely powerfull tower. two, its only a tower, make sure you give it ample support and put others around it so that it doesn't get destroyed and you lose all your precious men.
at the starting place hold alt + shift and hit A
then in the game hold alt and hit X to get 100 popularity and 1000 gold hold alt and hit K to make all building free
When a trebuchet or catapult runs out of stone simply select the 'launch cow' option and click as if to attack an archer or defensive unit. Now a large circle of torches will appear. Click on that same man again and the circle becomes smaller. The siege unit now launches a rock instead!