What checks the power in a limited monarchy?

Every monarchy is a product of the the culture and political traditions of a specific nation and peoples. If a monarch acts within the limits set by his or her culture, the monarch increases his or her legitimacy and power. If he or she chooses to disregard the the customs and traditions of the nation, that same legitimacy is squandered and may in result in replacement and even the disestablishment of the monarchy as an institution. While the details differ from country to country a monarch is effectively checked by the threat of loss of power position and esteem.

By custom the British monarch reigns but does not rule. Political power is vested in Parliament and the monarch's power lies primarily with the great respect the British peoples hold for the monarch. For instance the British monarch theoretically has the power to veto legislation, but it is generally agreed that this power would be rescinded should it ever be exercised.

The Spanish monarch by contrast is a constitutional monarch who rules under a constitution ratified by the Spanish people in the late 1970's. As with most of the rest of the modern western world the legitimacy of any government lies in the consent of the governed to be governed.

The monarchs of Morocco and Swaziland both claim and to a great extent exercise absolute power. Even these monarchs are effectively checked by cultural/religious expectations and the spread of western style ideas of legitimacy.

One constitutional monarchy which has effectively tended and grown the legitimacy of their position are the Princes of Liechtenstein. In 2003 the Princely house of Liechtenstein was actually increased by constitutional reform.