Monday, 11 March 2019

Building a Castle: Defensive Towers

Caerphilly Castle
Early castles had walkways along the top of the walls on the inside, but it was soon realised that towers provided much better defences. Not only did they stick out from the walls, providing better angles to shoot attackers, but arrow slits or loops were built in at each floor level to enable archers to shoot without getting shot at.

Initially towers were open on the inside, but later the back was built and provided extra accommodation. Towers were rectangular or hexagonal, but master masons found round towers more stable, especially since a major technique in attacking a castle was to undermine the walls or towers. Providing more defences was no good if the towers could be undermined and toppled. Some castles had D-shaped towers built onto the existing walls.

Gu├ędelon Castle
The masons at Gu├ędelon found it difficult to find the centre of the tower when laying out the plan for the foundations, until they marked the continuation of the two walls and the corner where they met was the centre of the tower. The tower was marked out using a pole in the centre with a horizontal bar attached, the length of the tower’s diameter. A plumb line hung from the end of the bar marked a perfect circle when the pole was turned.

Arrow Loops
Arrow loops Caernarfon Castle (
An arrow loop is a vertical slit in the wall (typically 60-120cm high) which is narrow on the outside (typically 5-10cm) and wide on the inside to allow the crossbowmen room to move. Any arrow which entered the slit must have been by chance. Crossbows were used in castles instead of longbows because they were easer to use in the confined spaces and delivered a bolt powerful enough to pierce armour.

Plantagenet arrow loops had a horizontal bar half way up, making the shape of a cross, which gave a wider field of vision. There was often a step below the loop for the crossbowmen to put their foot on to give them a steadier aim.

[adapted from The Medieval Castle Haynes Manual by Charles Phillips]

Ann Marie Thomas is the author of four medieval history books, a surprisingly cheerful poetry collection about her 2010 stroke, and the science fiction series Flight of the Kestrel. Book one, Intruders, and book two Alien Secrets, are out now. Follow her at

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