Stone wall towers, especially in age 2, are considered fairly strong. There is however a more subtle bug with this: the stairways for infantry can be used when the outline of tower is placed, rather than when the post is constructed.
I.e. only the placement of the stone tower, not actually building it, already allows infantry to reach the top of the wall, leaving enemies no option to follow them.
Considering it is unrealistic to walk stairs that do not exist and the already powerful stone wall push in age II this must be a bug.