Noun
(military) A body of troops comprising the chief guard units, especially a unit of cavalry on the wing of a camp towards the enemy, or a group of soldiers in a fortress responsible for dealing with disturbers of the peace. [from 17th c.]
(fortification) The keep of a castle; a guardhouse or building in a fortress where such a body of troops is lodged. [from 17th c.]
Source: en.wiktionary.org