The Palace of the Popes is the most important secular Gothic building in central Italy and the defining monument of Viterbo. Construction began in 1255 and was completed in 1266 to house the papal court after it relocated from the violence of Rome. The palace's great loggia — seven arched bays open to the sky above the valley — is an extraordinary work of Gothic architecture, elegant and austere at once.
Five papal conclaves were held here. The longest began in 1268 after the death of Clement IV and lasted 33 months, becoming the most protracted conclave in Church history. The Viterbese finally locked the cardinals in, stripped the roof off the hall, and reduced their meals to bread and water. Gregory X was elected in 1271, and the experience prompted him to codify the conclave rules still used (in adapted form) today — the origin of the word "conclave" (cum clave, "with a key").
Today the palace hosts the Museo del Colle del Duomo, which preserves papal artefacts and medieval sculpture. The loggia and the great hall can be visited; the views from the loggia across the valley are among the best in Lazio.