Genesis 4:17

And Cain knew his wife; and she conceived and bare Enoch. Since of one blood all nations of men are made to dwell on the earth, it was necessary, in the early days, for brothers and sisters to marry. Later, the Lord Himself changed this order, the marriage of close relatives at this time being apparently also against a law of nature. Cain called his first son Enoch (dedication), since he believed that his generation would be built up through this son. And he builded a city, and called the name of the city, after the name of his son, Enoch. Literally the text has it that Cain was building a city, that is, a fortified enclosure, as a stronghold for his entire family; it was the work of a lifetime, and gave him some sense of security, affording him a shelter whenever his restlessness permitted him to return home.