One of the four residences used by the Bourbon Kings of Naples during their rule of the Kingdom of Two Sicilies (1730-1860). The Royal Palace is on the site of an earlier building meant to host King Philip III of Spain, who however never made the trip. The architect chosen for that palace was Domenico Fontana. The building was put up on the site of an even older Spanish viceroyal residence from the early 16th century. The 17th-century palace visible today is the result of numerous additions and changes, including some by Luigi Vanvitelli in the mid-18th century and then by Gaetano Genovese.