Castel Gandolfo occupies a volcanic ridge above Lake Albano in the Alban Hills, a landscape valued since antiquity for its cooler air and proximity to Rome. The area lay near the legendary Latin center of Alba Longa, and Emperor Domitian built an extensive first-century villa nearby. Its terraces, theater, cryptoporticoes, and gardens later became part of the papal domain. Medieval fortifications belonged first to local lords, including the Gandolfi, and then to the Savelli family, whose castle gave the town its enduring name.

Financial disputes allowed the Apostolic Camera to take possession of the Savelli estate at the end of the sixteenth century. Pope Urban VIII, who knew the region before his election, made Castel Gandolfo a formal summer residence. Architect Carlo Maderno adapted and expanded the old castle, and Urban became the first pope to vacation there in 1626. The palace offered distance from Rome's heat and disease while remaining close enough for government. Its restrained façade faces the town square, distinguishing it from the more theatrical country villas created by many noble families.

Successive popes reshaped the ensemble. Under Alexander VII, Gian Lorenzo Bernini designed additions and the domed church of San Tommaso da Villanova, giving the piazza a coordinated Baroque character. The Barberini family developed Villa Barberini across part of Domitian's ancient estate, while Villa Cybo and surrounding gardens were later joined to the papal property. Terraces, avenues, fountains, farmland, and archaeological remains created a residence larger than the palace alone. Papal households used the complex for rest, audiences, administration, scientific observation, and carefully staged encounters with local residents.

Political upheaval repeatedly interrupted this pattern. French occupation and the Roman Republic affected papal property, and after Italian troops entered Rome in 1870 the popes largely withdrew inside the Vatican, leaving Castel Gandolfo little used. The Lateran Treaties of 1929 reconciled Italy and the Holy See and granted the papal villas extraterritorial privileges. Modernization followed. Because Rome's illumination interfered with astronomy, the Vatican Observatory moved instruments to Castel Gandolfo in the 1930s, continuing a church-sponsored scientific tradition in new domes above the palace and gardens.

During the Second World War, the extraterritorial estate sheltered thousands of civilians escaping bombardment and fighting around Rome. Refugees crowded halls, gardens, and farm buildings; dozens of babies were born there. The palace itself was not immune from the war's danger, and bombing in the area caused casualties. After 1945, popes again used Castel Gandolfo as a summer refuge. John Paul II added a swimming pool, and Benedict XVI returned after his 2013 resignation for a short transitional stay, reinforcing the site's association with the private rhythms of the modern papacy.

Pope Francis chose not to spend summers at the palace. He opened the gardens to visitors in 2014 and the papal apartments as a museum in 2016, shifting much of the complex from exclusive residence toward public interpretation. Later popes may alter the pattern again, as the villas remain functional Holy See property rather than a retired monument. Castel Gandolfo's significance comes from the overlay of Roman imperial remains, a feudal town, Baroque architecture, papal diplomacy, scientific research, wartime refuge, agriculture, and tourism within a landscape still shaped by active religious government.