Shell Keep castles were a mid-Medieval innovation in castle design and defence. They’re relatively uncommon, and so attract a good deal of attention today.
A ‘Shell Keep’ is the name given to a thin, usually round, defensive enclosure that was built atop of the castle Motte (the man-made mound).
Crucially, these enclosures were built of stone. They replaced the original palisade fences of Motte and Bailey castles, which were built of wood.
Shell Keep castles are viewed today as an interesting, and often extremely picturesque, form of castle design. They mark an interim stage of development between the original Motte and Bailey structures, and more advanced stone castles.
Some great examples of this type of design are Restormel Castle in Cornwall, UK; Wiston Castle in Wales, UK; and I’d also include Clifford’s Tower in York, UK. All are pictured on this page.
How the Shell Design Solved the Problems of Motte and Bailey Castles
Shell Keeps solved some of the problems of early Motte and Bailey designs – but they were, by no means, perfect.
First, let’s explain some of the problems and limitations of the early-Medieval Motte and Bailey design. Primarily, it was typified by its use of simple materials and straightforward building techniques.
On top of the Motte (the man-made mound), defendants typically built wooden towers and wooden palisade fencing to protect the castle. They also used this fencing to protect the Bailey – an adjacent, raised area with a flat top, which housed residential buildings.
Although wooden defences had significant advantages (primarily that they were both quick and cheap to build) they had one fundamental flaw. They were extremely vulnerable to fire. Many early Motte and Bailey castles were burned to the ground with flaming arrows.
In the early 1100s, engineers and architects came up with a unique solution to these problems. The Shell Keep castle was born.
The Characteristics of Shell Keep Castles
The simple idea behind the Shell Keep was to re-wrap an existing castle within a jacket of stone. Rather than go to the vast expense of rebuilding the innards of the castle, Shell Keeps replaced the wooden outer wall which had previously encircled the castle.
This meant that the Shell simply contained the existing wooden buildings, and acted only a shield against attackers. It was an refinement of an existing castle design – and not a new form of castle construction.
Shell Keeps tended to be oval, circular or polygonal in shape; and completely encircled the wooden towers and buildings of the castle. Some included simple gateways or towers, although these weren’t the norm.
Originally, these Shell Keeps had little communication with the contents of the castle – the new walls bore little structural relationship to the wooden components within.
However, over time, castle-inhabitants began to build new wooden buildings which backed onto the stone Shell walls.
The Disadvantages of Shell Keep Designs
Most Shell Keeps were relatively small in size – no more than 25-50m in diameter – and encircled only the top of the Motte (rather than wrapping around the base of the hill, too). They tended to be light, short constructions, with relatively thin walls.
Why were they so lightweight – after all, weren’t they meant to strengthen the castle and improve defence?
“The worry was that a Shell Keep could cause the man made mound to subside, or even collapse”
Well, it was all because medieval engineers approached Shell Keeps with a great deal of caution.
The mound of the Motte was often man-made (that is, an artificial pile of earth rather than a naturally-formed hill). Their concern was that a heavy shell upon a man-made Motte would cause the hill to subside, and everything on top to collapse.
Their concerns were probably bourne out by bitter experience. Modern excavations of older Mottes have shown many to be seriously unstable. Perhaps some builders started out constructing shell keeps, but had to abandon their work.
The decline of the Shell Keep
Shell Keep castles were relatively short lived – few were ever built, and the style faded after the 1210s.
After this time, Motte and Bailey designs had truly faded from fashion, being replaced with fully fledged stone castles – the next stage of castle development. Of course, without Motte and Bailey castles, there was no need for Shell Keeps.
In total, English Heritage estimates that there are around 70 of these castles that still exist in the UK.
Shell Keep Castles That You Can Visit Today
The most notable – and most famous – remnants of a Shell Keep castle is Restormel, in Cornwall, UK.
Restormel is a perfectly-round (and surprisingly symmetrical) example of the form. The original Motte and Bailey castle was built in the early 1100s; and the stone work was added in the early 1200s.
The walls of Restormel are around 2.5m thick and 8m tall – consequently quite a beefy size for a true shell-keep; but the castle was expertly designed for defence, as it boasted a comparatively very deep moat. There’s more about Restormel Castle on the English Heritage website.
Another excellent example of a Shell Keep castle is Clifford’s Tower, which is in York (again in England).
Clifford’s Tower is the last remaining part of old York Castle. It’s an interesting little tower, as it’s shaped like a four-leafed clover, and enjoys great views across York city. It’s one of the two castles in York.