 The major change occurred six years after work began when the seven deep chapels around the choir opening off a single ambulatory were turned into shallow recesses opening off a double-aisled ambulatory.Australian architectural historian John James, who made a detailed study of the cathedral, has estimated that there were about 300 men working on the site at any one time, although it has to be acknowledged that current knowledge of working practices at this time is somewhat limited.Normally medieval churches were built from east to west so that the choir could be completed first and put into use (with a temporary wall sealing off the west end) while the crossing and nave were completed.