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Building a Castle: The Shell Keep

If you are one of the aristocracy and your headquarters are on a man-made hill behind a wooden palisade, you’re going to be looking for something better, especially if it has burned down once or twice. The answer is to use stone.

The first keeps were a stone wall enclosing the top of the motte in a motte and bailey castle. The Normans were afraid the man-made motte was not strong enough to support a stone building, but a shell keep spread the weight and allowed the buildings inside to be made of wood. All the most important rooms, particularly the lord’s chamber, were built inside. Should the outer walls be breached, attackers had another wall between them and their goal. And a much bigger one.

Shell keeps had deep foundations (1.8-2 metres (6-6.5 feet)) and the walls were thick (3-3.6 metres (10-12 feet)) and high (4.5-9 metres (15-30 feet)). An example of a shell keep is the round tower in the middle of Windsor Castle, though changes have been made since. Most shell keeps were built during the 11th and 12th centuries. A favourite shape was polygonal, sometimes with a curved inside.

Wiston Castle shell keep in Pembrokeshire is polygonal, with 18 sections on the outside and a curved inside. There was an arched gateway and the cavities for the bars across the gate can still be seen. The bailey enclosed by the keep is 18 metres (60 feet) across, where the buildings of the castle – including the hall and chapel, barracks, stores and guests’ accommodation, built of wood – would have stood.

Restormel Castle in Cornwall was built originally as a motte and bailey fortress around 1100, and converted to a shell keep in the late 12th or early 13th century by Robert de Cardinham. It’s walls still stand today, 7.6 metres (25 feet) tall and 2.4 metres (almost 8 feet) thick. The bailey is 38 metres (125 feet) in diameter and the shell keep is enclosed by a vast ditch 15 metres (50 feet) across and 4 metres (13 feet) deep.

Cardiff Castle in Wales has a 9 metres (30 feet) stone keep which was built around 1150 on top of the motte that was possibly built with a wooden structure by William the Conqueror himself.

[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