The swiss hall at the pope's summer residence in Castel Gandolfo.The gallery of Alexander VII at the pope's summer residence in Castel Gandolfo.The Papal Palace of Castel Gandolfo (or the Apostolic Palace; le palais des papes; Papstpalast ) is a 55-ha complex of buildings in a garden setting in the city of Castel Gandolfo; Italy; including the principal 17th-century villa; an observatory and a farmhouse. The main structure; the Papal Palace; has been a museum since 2016. It served for centuries as a summer residence and vacation retreat for the pope.Castel Gandolfo; Italy. 2016.