The Keep incorporates the remains of the original medieval castle of the Maguires.
Some of the ground floor masonry including the vaulted ceilings probably belonged to the earliest Maguire tower-house. Some medieval slit windows and the remains of a garde robe and latrine survive in the upper levels.
After the defeat of the Maguire chieftains, the Keep was rebuilt in the early 17th century by its new constable, William Cole, to house a garrison close to where Cole was to found the plantation town of Enniskillen.
In 2007 & 2008 the Castle Keep underwent a major refurbishment. The new displays are now open.