Rome is unique in history as a single city that became a world empire. The Roman Empire extended its power by planting new cities, a practice that made land surveying and city planning essential as part of the imperial process. After the fall of the Roman Empire, a period of infamous debauchery and decay, Rome was rejuvenated and rebuilt by the Catholic Church as a symbol of the empire of Christianity. Historic architectural ruins were combined with Baroque city planning to establish an urban model widely emulated in the grandiose planning of other European cities. Today Rome is more of a symbol than a functioning city‑most of its economy is derived from tourism that celebrates its past, rather than its future.