"A man without self-control
is like a city broken into and left without walls. (ESV)
Consider with sadness the city with no wall against marauders or brigands, the traditional bane of history and geography. Simply to withstand a minor attack, even if help could arrive shortly, would take great effort if there are no walls for the defenders to stand behind. There is no margin for error. Can guards stay alert - 360 degrees - through the full dark of a Middle-eastern night 3000 years before electricity? And doesn't the lack of walls increase the odds they would have to? That is, would not the armed bands come specifically to the city that men say has had its walls broken? Wealth to steal, women to rape, healthy young people to sell into slavery. In the Judea of that day such a city is probably doomed. It's just a matter of time.
But it's a simile. The proverb is giving advice about men, not cities. A man without self-control has less wealth than he could, less education, and fewer accomplishments that might lead to building a successful business or rewarding career. His health is poorer than it should be. Misfortune of any sort will find him with little defense. He has difficulty climbing out of such a hole because how can a long, hard, slow climb be made without the very self-discipline he lacks? And his weakness can be seen from a distance. Because he cannot be depended upon, his circle of friends is shrunken and his reputation is not good. He is vulnerable and others know it. Likely they talk about it. Word gets around. And the sad truth about humanity is that any particular vulnerability attracts exactly those that can best exploit it.
Like a city with broken walls attracts the marauder.