The largest Gothic palace ever built, the Palais des Papes was built by Pope Clement V, who abandoned Rome in 1309 as a result of violent disorder following his election. It served as the seat of papal power for seven decades, and its immense scale provides ample testament to the medieval might of the Roman Catholic church. Ringed by 3m-thick walls, its cavernous halls, chapels and antechambers are largely bare today, but an audioguide (€2) provides a useful backstory.
Highlights of the palace include 14th-century chapel frescos by Matteo Giovannetti, and the Chambre du Cerf with medieval hunting scenes. It can be hard to take it all in, so it might be worth joining one of the regular guided tours run by the tourist office.
The palace has been inscribed as a World Heritage site by Unesco since 1995. Look out for special events during the Festival d'Avignon, and special son et lumière (sound and light) shows from mid-August to September.