The current cathedral is located on the foundations of the former basilica raised in the 6th century by bishop Delmas. In 1277, Raymond de Calmont d'Olt laid down the first stone of the current cathedral. The first building campaign began in the 13th century and finished one century later. But diseases, wars and lack of money stopped the work.
In the 15th century, architect from Lyon, Jacques Morel, raised the nave, finished 100 years later. The bell-tower dates back to 1513, it's the current one! They also rebuilt the roof, the choir and the nave's vaults in 1625.
By the way, when came the French Revolution, they decided to demolish the bell-tower... But Montagnards came in the city and calmed things down: they didn't destroy the tower but replaced the saints' statues by portraits of Marat, Pelletier or Chalier. When the episcopal see was back in 1823, the cathedral was restored.