The historic Castello district of Cagliari perches atop a limestone hill overlooking the gulf, but its most intriguing feature lies underground. Beneath the modern city streets lie multi-layered historical strata from Phoenician, Roman, Pisan, and Aragonese eras. Descending into subterranean crypts and tunnels cut into the limestone reveals ancient Roman quarries that were repurposed as air-raid shelters during World War II. This vertical layout turns historic Cagliari into an upside-down historical skyscraper where traveling deeper means traveling back in time.