Skandham-5. Chapter-1. Slokam-18.

One who is situated in household life and who systematically conquers his
mind and five sense organs is like a king in his fortress who conquers his
powerful enemies. After one has been trained in household life and his lusty
desires have decreased, he can move anywhere without danger.

The Vedic system of four varnas and four äsramas is very scientific, and its
entire purpose is to enable one to control the senses. Before entering
household life (grhastha-äsrama), a student is fully trained to become
jitendriya, a conqueror of the senses. Such a mature student is allowed to
become a householder, and because he was first trained in conquering his
senses, he retires from household life and becomes vänaprastha as soon as the
strong waves of youthful life are past and he reaches the verge of old age at
fifty years or slightly more. Then, after being further trained, he accepts
sannyäsa. He is then a fully learned and renounced person who can move
anywhere and everywhere without fear of being captivated by material desires.
The senses are considered very powerful enemies. As a king in a strong fortress
can conquer powerful enemies, so a householder in grhastha-äsrama, household life, can conquer the lusty desires of youth and be very secure when he takes vänaprastha and sannyäsa.