The Bridge at Avignon (above) is famous because of a popular song, but in olden times it was important for far more serious reasons.
The only fixed bridge over the Rhone south of Lyons, in the early Middle Ages Avignon Bridge marked the boundary between the Papal State (Comtat Venaissin) and France. From 1309 to 1377 Avignon was the official residence of the Pope and a great palace (right and bottom) was built there. (This period should not be confused with the Schism of 1378 after which there were two men claiming to be pope, one of whom resided at Avignon and the other in Rome.)
Relations with France were not necessarily friendly in the early years of the Avignon Papacy. Later French influence grew. The Bridge was guarded and fortified by both sides, though at some stage apparently song and dance parties were held on an island which formed the base of some of the bridge's great stone piers. What appears to be the far bank of the Rhone in the photograph below left is actually a large island; there is a further channel beyond which is not visible. So the song has it wrong, the dancing was sous and not sur le pont.
The papal era bridge had 22 stone arches, but was vulnerable to damage by the Rhone's powerful floods which would sweep away some sections. It appears temporary patches were made with wood, but after being finally abandoned in the 17th century much of the bridge was demolished or swept away, so that now only four arches remain, into one of which the Chapel of St Nicholas is built.
Modern archaeology has discovered Roman foundations and radiocarbon dating suggests that a Gallo-Roman bridge may have existed for a time between 290 and 530 AD.
The canonised shepherd boy Saint Bénézet inspired the building of a bridge in the 12th century by miraculously lifting a huge stone. He was originally interred in the St Nicholas Chapel on the bridge itself. After the abandonment of the bridge, his relics were removed to the Hôpital du Pont on the Avignon side. Saint Bénézet's original bridge was destroyed during the Albigensian Crusade, when the Cathar Heresy (so-called) was extirpated in Provence.