The Romans were the first to build a citadel on Toledo’s highest hill during the 3rd century. It was modified by Alfonso VI of León and Castile at the end of the 11th century and again by Alfonso X of Castile during the 13th century. In 1545, while Charles I ruled the Spanish Empire and was also Charles V, Holy Roman Emperor, he commissioned architect Alonso de Covarrubias to convert the Alcázar into a residence. All of these modifications explain why the Alcázar bears so many architectural styles. The former fortress now serves as the National Army Museum and the Castilla-La Mancha Regional Library.