anyone have any suggestions on how to best assualt a castle?

in my first game i took my first castle easily, the 2nd just barely survived alone, and in my new game i cant take one at all.

i sit there with my seige tower as it moves real slow and alot of soldiers are killed before the ramp even drops, then as i get up top im being shot at from the front and sides so i cant block it.

so whats a good weapon to use in this assualts?