I should have been clearer. I can recognize the advantage of walls in single player games. AI has difficulties in assaulting defended settlements. It makes basic mistakes like not placing siege engines optimally and then trying to drag them to a better position oblivious to tower fire. But this benefit is not due to the walls but dumbness of the AI. In the reversed situation – human attacking AI – the walls often provide benefit to the attacker (at least in my experience). It is true that walls create chokepoints, however, you already have “natural” chokepoints in every settlement: entrances to the plaza.

It would be interesting to know how much the walls are valued in multiplayer battles.