posted 03-11-2004 09:29 AM
A shell keep was usually found in an upgraded motte and bailey castle where the top of the motte was simply walled in to enclose the living structure. So really the insides of it would be open ground with a building or two in it.A square keep tended to be a tall Tower structure with 2-3 floors.
Here is a site that gives a good definition of what a Keep is as well as the different types. It also includes a cut-away illustration of a square Keep. The site itself has a large amount of castle info including links to a number of English Castles
http://www.castles-of-britain.com/castlesj.htm
Good Designing..