Definition of "donjon"

[...] 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.

2007, Michael Chabon, Gentlemen of the Road, Sceptre, published 2008, page 132

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.

1819, Walter Scott, Ivanhoe