A tower was a tall structure on the outer wall of a castle and most castles had more than one. None of them was as tall as the keep. Despite that, they were very much a sign of dominion in the early, Norman, castles. They were called donjons, from the Latin ‘dominor’ – to master, to rule over.
Originally towers were square. This made them fairly easy to build. Square towers were, however, vulnerable to mining. If the besieging army could send miners underground, they could cause a tower to collapse by setting fire to the wood that had been supporting the tunnel under a corner of the tower. The corners were weak spots.
Round towers were less vulnerable to undermining. In addition, it was more difficult to rest a ladder against a round tower, which meant that castles with round towers were less easy to storm.
When Edward I built his showpiece castle at Caernarfon, he built polygonal towers inspired by the towers of Constantinople.
Towers had arrow loops built into them allowing the defenders to shoot arrows or crossbow bolts at attackers whilst having some form of protection. The shape of the arrow loop allows the archer, or crossbowman, to cover quite a lot of the ground below the tower. There is room for them to move from side to side and up and down, but the part of the arrow loop presented on the outside is so narrow that opposing archers or crossbowmen would need a lot of skill to get an arrow through it.
This is the same tower from the outside. You can see how narrow the arrow loops are. Any soldiers attacking this tower would have to shoot upwards as well. In this particular instance, they’d probably have to do it from a ship, as the water used to come up to the walls.
Ideally a castle would have enough towers around its curtain wall to allow archers and crossbowmen to cover all of the ground outside and inside.
Archers and crossbowmen were not the towers only means of defence. Mangonels and other forms of catapult could be used from the tops of towers. This would give them a greater throwing distance than similarly sized catapults used by the besiegers.
This last tower is from a bishop’s palace. Despite its name, Wolvesey Castle isn’t a castle, although it was fortified. It was built by Henry of Blois, Bishop of Winchester, in the mid-twelfth century. He was the brother of King Stephen, whose reign was mostly taken up with the civil war known as the Anarchy. Somewhat surprisingly, given that he was the king’s brother, Henry changed sides more than once, necessitating the fortification of his palace in Winchester.