This is a city carved by walls. What does this joke mean?

The joke "That's a city carved on the wall" means that the infrastructure and buildings of this city (usually a small town or village) are very simple, without careful planning and construction, but they are hastily built against the wall, so they look rough.

The humor of this joke is that it depicts the image of a city with imperfect infrastructure and poor building quality in an exaggerated way, which leads people to sneer and sneer at the living and working conditions of this city.

It should be noted that this joke is not necessarily malicious, but may just be intended to express views on urban construction and urban life through exaggeration and ridicule. However, in some cases, it may also be used to ridicule or belittle certain cities or regions. Therefore, when using this joke, we need to consider whether the specific context and occasion will hurt the feelings and interests of the parties.