Castel Gandolfo (kästĕl` gändôl`fō), town (1991 pop. 6,784), in Latium, central Italy, in the Alban Hills, overlooking Lake Albano. Possibly occupying the site of ancient Alba Longa, it is the papal summer residence. The papal palace (17th cent.), its magnificent gardens, the Vatican observatory (founded 1936), and the Villa Barbarini enjoy extraterritorial rights. The Church of St. Thomas of Villanova was designed (17th cent.) by Bernini.