The city is a big concept in Scripture. It’s cast in a somewhat negative light before the flood, but afterward is pictured as much more promising for human flourishing, culminating in the heavenly city of Revelations. The city of Jerusalem was most significant for God’s people because God himself dwelled there. More than that, God personally built and protected Jerusalem, until his people rejected him and were exiled, and the city was destroyed. The implication of the metaphor if this Proverb is not just people are cities but more specifically God’s people are God’s city.
We can say the metaphor expands to say self-control is a city wall. “Self-control” is a paraphrase of the Hebrew, which says something like “restraint of spirit”. Implicit even in this expression, then, is the idea that the spirit has a structure. And apparently that structure can be damaged, which is distinctly a bad thing. Put conversely, whoever lacks control over his spirit—whose spirit is damaged and left in disrepair—is as good as ruined. The one without an intact spirit is already “broken into” and left as exposed as he was found.
