It would appear that the daring scale of the monument's architecture (54m long, 15m wide, with a 12m central nave) and the lack of any exterior buttresses on the north and south walls, led to its partial collapse in three different earthquakes, in the early 14^th, early 16^th and 18^th centuries. The last of these led to the collapse of: 1) the roof; 2) the greater part of the north wall; 3) the 14^th century chapel in the northeast; 4) the 15^th century chapel in the southwest; 5) the east groin vault and part of the rose window; 6) the northwest buttress pier; and 7) the upper section of the west wall.