T. Hughes.
4. That which keeps or protects; a stronghold; a fortress; a castle;
specifically, the strongest and securest part of a castle, often used
as a place of residence by the lord of the castle, especially during a
siege; the donjon. See Illust. of Castle.
The prison strong, Within whose keep the captive knights were laid.
Dryden.
The lower chambers of those gloomy keeps.
Hallam.
I think . . . the keep, or principal part of a castle, was so
called because the lord and his domestic circle kept, abode, or
lived there.