How important was a medieval queen?

There were a fair number of queens and empresses who ruled in the Middle Ages. Empress Irene was ruling the Byzantine Empire when Charlemagne was crowned in Rome. There were a number of female Spanish monarchs. A woman named Hedwig was crowned King of Poland (yes, king). One of the most successful monarchs of her time was Queen Margaret I of Denmark.

There is a link to the European sections of a list of queens and empresses regnant below.