Rule by oligarchy

"[William] the Conqueror had gathered the central power into his own hands, but even he needed ministers to conduct his government efficiently."

Except for the very smallest local situations in which a man can terrorize the population by his own physical force, every form of government is essentially an oligarchy. Since he cannot do everything by himself, the top man must have a small group of wealthy or powerful people who will support him. And, the ruler must have the support of the military leadership. In order to keep the military in line, he must have the support of a secret service or Gestapo or KGB. In free countries, the candidate must have the support of party leaders. So, in each form of government, it boils down to the rule of the few, or an oligarchy.