What city was the center of the medieval Papal States?

The story behind the answer

As the seat of the bishop of Rome—the pope—the city served as both the spiritual center of Western Christianity and the administrative heart of the Papal States. From the 8th century, after the Donation of Pepin (756), the papacy exercised temporal rule over swaths of central Italy from this base, blending religious authority with princely governance.

Although popes resided in Avignon from 1309 to 1377, their sovereign territories remained in Italy, and the return reaffirmed the city’s primacy. Key moments, like the crowning of Charlemagne in 800, underscored its centrality in medieval geopolitics. The Papal States endured until Italian unification in 1870, when they were annexed; the Lateran Treaty of 1929 later defined the modern microstate that preserves a remnant of this medieval temporal power.