Keep
The Cyclopedic Law Dictionary · Walter A. Shumaker and George Foster Longsdorf; ed. James C. Cahill · 1922
The Cyclopedic Law Dictionary
A strong tower or hold in the middle of any castle or fortification, wherein the besieged make their last efforts of defense, was formerly, in England, called a "keep"; and the inner pile within the castle of Dover, erected by King Henry II. about the year 1153, was termed the "King's Keep"; so at Windsor, etc. It seems to be something of the same nature with what is called abroad a "citadel."
Jacob.