Foolproof Tower is located on the left side of the last bit of the approach to the Wonderland Valley (see that approach map for more info). Just before the valley broadens, you'll spot the formation on the left. Curl around to the west and then south to scramble up to the base of the wall. A more direct approach from the wash w/scrambling may be possible as well. More often than not, the path of least resistance is the easiest, if not the shortest.