A shell keep is a type of medieval fortification consisting of a circular, oval, or polygonal stone wall built on top of a castle’s motte (a raised man-made earth mound). Popular in the 11th and 12th centuries, these stone walls served as an upgraded "shell" to replace the vulnerable wooden palisades of early motte-and-bailey castles.

Key Design Features:

Central Courtyard: Rather than a single massive structure, shell keeps enclosed a small, open inner courtyard at the top of the mound.

Lean-to Buildings: Instead of stacking rooms vertically in a large tower, castle inhabitants built residential buildings, halls, and kitchens backing onto the inside of the outer stone wall.

Structural Safety: Early Norman engineers were hesitant to build heavy, multi-story stone towers on top of artificial earth mounds, fearing the weight would cause the motte to collapse. A shell wall was lighter, better distributed, and protected existing structures from fire without risking the mound's structural integrity.

Wall-Walks: The stone tops featured crenellations and wall-walks for archers and defenders.

Access: Entry was typically gained via a straight flight of stairs running up the side of the motte, leading to a simple arched doorway or a fortified gateway.

Notable Examples:

Restormel Castle in Cornwall, England is widely considered the best-preserved and most classic circular shell keep in the world.

Clifford's Tower (the only remaining part of York Castle in England) is an iconic, four-leaf clover-shaped shell keep.

The Round Tower at Windsor Castle in Berkshire, England is a famous example, originally built as a shell keep and remodeled heavily in later centuries.