Hello all. It might seem like a really mundane question, but it just struck me. I always used to think that a city didn't typically have walls all around it. That, be it Roman, be it Norman or be it a Napoleonic fort, that the city "walls" were an inner ring, that encased the upper class and palace, senate, whatever the form of government/royalty might be, and that outside the walls, the rest lived, and were more or less sacrificed in the event of a protracted siege/assault.
So, first, how true is the above? I'm currently watching Gladiator, which I know isn't a source in itself, but the city of Rome was surrounded by perimeter walls, and I realized that, some places in history DID surround their people with walls, and left the farms outside. But.. how did they get around expansion? Populations expand, houses need to be built... did they keep shifting the walls? Did they have development programs every so often to build new sections?
Obviously, this differs from place to place, and from era to era, so hit me with whatever time period you know of. It's all interesting to me, and it all helps answer me question.
They didn't try to keep up with the population. Most of the important buildings were stationed behind the walls so as the population grew larger, houses were built outside of the walls' reach.
First, your conjecture in the first paragraph isn't true. Cities usually enclosed the whole population inside their walls, although many had an "inner ring" called Citadel, where the defenders retreated in case the main walls were breached, but that was something of a last resort.
Second, the answer to your question regarding population growth is also mundane - population in the middle ages simply didn't grow as fast as it does today. The population growth was very, very steady and the population sometimes decreased as a result of famine or plague. If the cities grew, they mainly solved it with a higher density development, or alternatively they built new city walls, such as in the case of Constantinople. But again, this development happened over an extremely long period, in the beginning Constantipople quickly expanded and then kept the new walls for many hundreds of years until its fall.