Genesis 4:17

His wife. She was a daughter of Adam, and Cain's own sister; God dispensing with such marriages in the beginning of the world, as mankind could not otherwise be propagated. --- He built a city, viz. In process of time, when his race was multiplied, so as to be numerous enough to people it. For in the many hundred years he lived, his race might be multiplied even to millions. (Challoner) --- The Hanuchta, which Ptolemy places in Susiana, (Calmet) may perhaps have been built after the flood, in the same place. Josephus says, Cain was the first who fortified a city; designing it for a retreat, where he might keep the fruits of his robberies, Antiquities 1. 3. Peirere founds his ill-concerted system of Preadamites, or of men existing before Adam, on the history of Cain exercising husbandry, building a city, &c.; as if there were any difficulty in supposing, that the arts would have made some progress in the lapse of above a century. (Haydock)