You're possibly misreading that.

In the Bible the city builder was Cain, and he built it because he was a criminal and needed protection from the world when he lost God's grace.

I.e., the message is that a city is a kind of protective exoskeleton or "safe space" when man becomes estranged from God (the Universe).

It's a story about man's fragility and ultimate futility.

That is a very idiosyncratic interpretation of the tables of patriarchs in Genesis.

A more orthodox interpretation is that Cain's line is the urban, civilization-creating line, you have metallurgy, music, cities, trade, all coming from that line. This is because Cain represented agriculture, whereas Abel represented cattle herding, thus nomadic, not rooted lifestyles.

After Cain killed Abel (e.g. sedentary agriculture replaced nomadism in most places), it was natural for Cain to build a city as he was not nomadic. It was not for "protection" from his fellow man, but merely the continuation of his choice to be a tiller of the soil rather than a wandering nomad. God's demand for Cain to be a "wanderer" in the land of "nod" ("literally 'land of wandering' in Hebrew") is thus the opposite of what Cain did -- he settled down and built a city. This creates a nice picture: Cain choose not to be a wanderer (nomadic cattle herder) but to be a farmer (settle in one place). God told him "You will be a wanderer" but then it says that Cain built a city. So is the message -- those who are rooted in one place on earth are spiritual wanderers, and those who wander are the rooted ones?

Of course everyone is free to read Genesis and get all sorts of meanings out of it, it is ultimately a personal exercise according to your own conscience, but when you say that your reading is the correct one and other people are "mistaken", then you better be able to back it up with some kind of consensus view.

FYI, more modern scholarship has noticed that the seven generations of Cain line up with the 7 "sages" in Sumerian accounts, these were the sages that came from the deep and taught the Sumerians agriculture, metallurgy, etc, and people have been exploring associations between the names of these sages and the names in the line of Cain.

Another interesting area of the study is that the lines of Shem are basically the same names as that of Cain, but Shem has three more figures. That certainly isn't an accident - the documentary hypothesis people say it is the same line, just copied over from different places, but that is an old view and I think a minority one. There are all sorts of discussions about that similarity.