 This population decline accelerated in the second half of the 20th century when significant numbers of Castel del Monte's residents migrated to Wallonia, many to work in its coal mines.Others migrated to France, Switzerland, the United Kingdom, and the United States.In recent years, former residents who emigrated to French-speaking countries and their children have been returning to Castel del Monte with more frequency for vacations and, in some cases, to retire.