Either side of the Cathedral, about half way along the north and south sides, there are two square towers. They were built between 1114 and 1133 as part of the Norman cathedral.
During the construction of the present cathedral, these towers were incorporated in the new building as transepts. This was a daring feat of engineering. A large portion of the south wall of the old north tower was removed and a massive supporting arch inserted. The same work was carried out on the north wall of the south tower. Several lower floors were removed from the towers and wooden ceilings inserted more than 20m (68ft) above the transept floors. This work was completed by 1326.
High up in the north tower is the ancient Peter Bell, which can be heard striking every hour, controlled by the clock mechanism below.
The upper levels of the south tower contain the ringing chamber and the bell chamber. The 14 bells housed here form the second heaviest peal in the world.
Exterior images of the North and South Towers: Diane and Malcolm Walker