The inner wall protects the prince; the outer walls settles the people.
The critical distinction here is between an impermeable defensive wall and an essentially aesthetic wall. The city wall only tells people where to build, not where to be. It defines the limit of permanent settlement, not the limit of human movement. It should be designed to make the city beautiful, not safe. City walls are in no way related to the nationalist border walls or flood walls of the early 21st century.
It is no coincidence that villa construction dominated the late Roman economy as aristocrats sucked capital out of the capital. Whether this caused the fall of Rome is unclear, but what is clear is that post-post-modern society is doing the same thing. The villa, or single-family home, represents maximum entropy in development. When everyone has their own home and garden, there is the least possible shared space. (Roadways are technically shared, but suburban roadways are minimally used, so essentially private at any given time.) It represents the least possible organisation, with one identical home next to the other, each owned by one couple of adults according to the division of domestic space and labor that has existed since before antiquity. The entropy here is both spacial and political. The spacial entropy is high because each unit has only one use, which is indistinguishable from the use of all the other units, while the political entropy is high because there are no collective decisions to be made: each property owner is responsible for one unit, and nothing else.
It is also no coincidence that the British suburbs voted overwhelmingly to leave the EU and that the US suburbs will vote for Trump. Democracy is a wave, and it is not immune from entropy. Urban gentrification isn’t happening because people have realised that suburbs are stupid. It’s happening because the suburbs in most places have reached maximum entropy, a point where new developments are so remote from any employment that there isn’t enough time in the day for the commute and people can’t make enough money to pay for it. At some point, new suburban development is economically untenable. Entropy is, after all, the loss of differentiation, whether thermal, informational, or spacial. People are excited by low entropy – the juxtaposition of high and low energy states – but there are two reasons to maximise entropy as quickly as possible. The first is serenity. Maximum entropy is quiet. The second is profit. You can make a ton of money from the rapid rise in entropy that comes with turning a farm field into a subdivision. Curiously, racist xenophobia and sexism seem coupled to this impulse to flatten the world with maximum entropy for reasons that are not entirely obvious.