The Royal Alcázar of Madrid was a fortress located at the site of today's Royal Palace of Madrid, Madrid, Spain. The structure was originally built in the second half of the ninth century by the Muslims, then extended and enlarged over the centuries, particularly after 1560. It was at this time that the fortress was converted into a royal palace, and Madrid became the capital of the Spanish Empire. Despite being a palace, the great building kept its original Arabic title of Alcázar.