When I build a wall that terminates at a cliff edge or otherwise unpathable terrain, predators and enemies are able to squeeze between the edge of the terrain and the wall. See screenshot below for example of raiders. This also happened to me on a different map with a bear sneaking around the end. In that one, the wall was at the bottom of the cliff instead of the top.
Edit: thought it would be useful to make it more obvious the path they took in the screenshot.
