Walls (or physical barriers) appear to be the standard defensive structure for a city in order to keep invaders out. Are there examples of cultures/cities that successfully defended themselves without the use of barriers, and how did they accomplish that? Are there examples of defensive tactics that relied on the attacker being within the city boundary?
Thomas Moore's "Utopia" describes a society that created relationships in order to forestall invaders, which is not the kind of situation I'm curious about. I'm curious about active defense against attack.
I wonder about:
- Trade cities
- Harbour cities
- Cities in areas without strong building materials (deserts, etc)