At least in the Middle Ages, there were algorithms that reigned over the king. A king had an executive role (comparable to a US president, greater than a parliamentary prime minister), but was constrained by customary law -- which is fancy-speak for "the way we've always done things," but even so, it meant there were things a king couldn't do. One of the issues Barbara Tuchman mentions in _A Distant Mirror_ is the way in which the French king could only levy a tax on the whole realm when at war, and even then couldn't continue it after the war ended -- the expectation being that "the king will live of his own," i.e. support himself from his personal estates, at peace.
Going forward in history, that's why the Carlists in 19th-century Spain were both pro-autocracy and fighting for their liberties: they preferred a king who was the absolute monarch of a half-dozen countries (Castile, Leon, Navarre, Galicia, Aragon, and Andalusia), instead of a centralized federal government in which none of the kingdoms had special privileges (note that Carlism was strongest in the northern parts of Spain) and all kingdoms' subjects were administered impartially from Madrid.
Going forward in history, that's why the Carlists in 19th-century Spain were both pro-autocracy and fighting for their liberties: they preferred a king who was the absolute monarch of a half-dozen countries (Castile, Leon, Navarre, Galicia, Aragon, and Andalusia), instead of a centralized federal government in which none of the kingdoms had special privileges (note that Carlism was strongest in the northern parts of Spain) and all kingdoms' subjects were administered impartially from Madrid.