siege was not used for riding in the past siege is more the thing of meele and ranged in battle cav was (is in crpg) kinda like a secret weapon which should bring the victory (kinda a special force)
but in siege they didnt have a place i mean they couldnt climb a siegetower (how they can in crpg) or going stairs up and down at least not that fast and strategly it wouldnt have been a good choice to choose cav