During a recent visit it struck me how rather unremarkable Avignon is otherwise. No port, and vulnerable to enemy attack from the south - why did he not rather pick Paris or some bigger city?
Technically, the Avignon popes resided first in Carpentras, northeast of Avignon, which was the capital of the papal territory known as the Comtat Venaissin and had been ever since it was given to the papacy by Philip III in 1274. Avignon proper was independent of the Comtat, but it was owned by the Angevin counts of Provence, who were allies of the pope and welcomed Clement V in 1309 as he sought a haven from the chaos of Rome (the Archbasilica of St. John Lateran had just been destroyed, and tensions were very high), a move which was suggested to him by Philip IV. Avignon was later purchased outright by Clement VI in 1348 from Joanna I of Naples.