Aw, I was waiting to read the whole thing before plugging Mortal Engines!
Anyway, I think a walking city can be really cool whether it's mechanical or magical/biological in nature. No matter what, it would provide protection to its citizens greater than a wall or other low-tech solution. Perhaps the city needs people to help it, and the citizens live in a symbiotic relationship? They fix it, feed it, steer it towards food, or something that it can't do on its own. So naturally people stayed on and now it's a little town!