Round stones were also used as part of the wall defence, they would be drop down a shot and mash into a group of attackers, just one of them rolling into troops could do a lot of damage, like if you had archers behind a woven weave shield defence, it would break them up and push the archer out into the open, the one that did not get hit that is, there is a castle in France that had tube shots built into the gate walls. The slope wall if it's the one I am thinking about, then that wall was build to serve the docks