donjon

päätorni The fortified tower of a motte or early castle; a keep.
2007, Michael Chabon, Gentlemen of the Road, Sceptre 2008, p. 132:

... the prison fortress called Qomr, a mound of yellowish brick rising up from the left back of the turbid river, in whose donjon by long tradition the warlord was obliged to lay his head.

It was a fortress of no great size, consisting of a donjon, or large and high square tower, surrounded by buildings of inferior height, which were encircled by an inner court-yard.