Only philosophers can serve as just kings because only this select group of people have knowledge of the Forms, a vague, otherworldly kind of absolute truth. Much like Huxley's Brave New World , Plato's ideal city gives total control to the government. This government, envisioned by philosopher kings and directed by their assistants, enforces a strict set of moral standards aimed at teaching virtue and goodness to the common people. Yet the ruling class doesn't live the life of luxury. Indeed many would consider it a very dull life.
Private property is curtailed severely, and possessing riches is strictly forbidden.
Plato takes these pains to ensure that the philosophers don't grow corrupt from amassing great quantities of wealth. Money, to Plato, is a concession to the base appetites of soul, which must be controlled. The other parts of the soul include the spirit, or devotion to honor, and the rational side, devoted to reason and logic. The rulers, or guardians of the people, are best able to find a balance between these competing interests, and thus are suppose to lead and educate the others. Plato argues that living justly is far superior to living unjustly because justice breeds happiness.