Ancient walls are pretty useful to build, because they give city strikes and dramatically increase a city's health. After that, though, does it feel odd to you that later walls follow a civ V system of improvement in damage strikes by era instead of wall level? City strikes can become really strong without investment in the late game, so I've never felt the need to create any medieval or later walls. It's also a bit odd that walls simply regenerate immediately when you capture a city in the late game, as though moving a tank through a city somehow strengthens the fortifications for the city being captured. It even rebuilds the wall if the city has been nuked, which is a bit ridiculous. It just feels like the devs started a really neat and useful city wall system in the ancient era, then didn't have time to implement it for the rest of the game, which I find to be a really weird design decision.