Why was colchester castle called a shell keep?

Colchester Castle is referred to as a "shell keep" because it features a stone outer wall that encases a central core or structure, resembling a shell protecting its contents. This architectural style emerged in the late 11th century as a means to provide enhanced defense while maximizing the use of available space within the castle. The design allowed for a more robust structure compared to earlier motte-and-bailey castles, combining fortification with residential and administrative functions. Colchester Castle is one of the best-preserved examples of this type in England.