@real_tabasco_sauce I was using Persians and these were stone walls, and I had developed the "Persian architecture" tech. The enemy simply "cut" the wall with roman swordsmen; more than one segment of wall btw. They were a lot of swordsmen, but still...
@Dizaka what you mention about the usage of walls as obstacles is true and I do it as well, but with palisades. Stone walls, on the contrary, are much more expensive and take longer to build, and usually you expect them to protect your city hermetically from whatever that comes without siege. Normally it works that way, ranged units cannot break down a wall, and this is the expected behavior. I was expecting the same for any kind of soldier units, but enough swordsmen can bring down walls.
In my experience, in multiplayer games (which I play a lot), the only ones that build stone walls are new players expecting they will protect them. I never see experienced players building stone walls.