Late in Henry’s reign, shell castles , or shell-keeps, appeared. High walls, sometimes 36 feet high, formed an outer shell on the top of the motte, sometimes in circular shape, sometimes polygonal. They enclosed the area at the top of the motte, but were not roofed.
Sometimes the motte was not stable enough to supposrt the weight of a keep and the walls, and in such a case, the walls were deemed more important than the keep. A flight of steps led up from the inner bailey to the shell-keep. Domestic buildings, roofed and built in the normal way, leant against the shell wall on both left and right, leaving a courtyard or garden in the centre space.
At Harbottle, the gatehouse defended the entrance to the inner bailey and therefore access to the shell keep. Towers strengthened the curtain wall, and the gatehouse was a twin-towered affair with a central passage over the actual gate and living accomodation above.
The lord’s hall was often built in the bailey, allowing for a more spacious building than could be built in the shell keep. A garrison might be housed inside the shell walls. The pic shows what is left of the massive gatehouse that once stood at Harbottle.