content: Walls are just intuitive at a very basic level, which arguably cannot be said for the wheel, which takes a bit of thought to come up with from scratch. According to Wikipedia the wheel was invented around 12,000 years ago while walls don't really have a set date of invention however they seem to go back just as far. Think about it, you don't want wild animals or invaders to be able to enter your settlement? Stack a bunch of mud and rocks up high enough all around it so that anything coming in is forced through a small chokepoint (gate) and boom! You have the first wall.