The Octroi Wall surrounded Paris and some outside towns like Franciade. The purpose was to allow the imposition of a tax on goods entering the city at all crossing points, the unstated purpose of which was to fight against foreign competition by increasing the price of imported goods.*
* Beating the tax gave birth to wall evasion which went on to become pole vaulting, which then evolved into pole dancing. If I've written it, it must be true!