The Cathedral of Notre-Dame des Doms stands on a rocky outcrop immediately above the Palais des Papes, older than the palace by nearly two centuries. Built in the 12th century in the Provençal Romanesque style, it served as the personal chapel of the Avignon popes during their 14th-century residence, and several popes are buried in the Gothic porch.
The cathedral is smaller and plainer than its position beside Europe's largest Gothic palace might suggest, but this contrast is part of its interest — you can trace the original austere Romanesque nave, the later Gothic additions, and the 19th-century gilded statue of the Virgin that crowns the bell tower. The interior holds a white marble episcopal throne and the tomb of Pope John XXII, who ruled from Avignon for 18 years and died here in 1334.
Entry is free. The cathedral is almost always overshadowed by the adjacent palace, which means you can usually walk in and look around in peace even when the Palais next door is heaving with tour groups.