How do you climb the tower in assassins creed 2 in altairs world

Holding "Right Trigger + A" will make climbing easier. Also, remember Rosa's technique of the climb jump by Pressing "Right Trigger + A" and then holding B to catch a ledge, etc. You can try switching sides as well, by moving to each 'wall' of the tower to look for new hand holds. Try those, see if it helps.