Shell keep

A shell keep is a style of medieval fortification, best described as a stone structure circling the top of a motte.

Castle engineers during the Norman period did not trust the motte to support the enormous weight of a stone keep.

A gazetteer compiled by archaeologist Robert Higham counted 21 shell keeps in England and Wales.

[1] Examples include the Round Tower at Windsor Castle[2][3] and the majority were built in the 11th and 12th centuries.

Clifford's Tower is often interpreted by modern visitors as a shell keep due to explosion damage, in 1684, which removed the roof and its central supporting masonry.

An aerial photograph of Windsor Castle, with three walled areas clearly visible, stretching left to right. Straight roads stretch away in the bottom right of the photograph, and a built-up urban area can be seen outside the castle on the left.
An aerial view of Windsor Castle with its shell keep (called "The Round Tower") prominent on its motte inside the middle ward (middle bailey)