donjon

noun

1. The fortified tower and main residence of a motte or early castle; a keep.

   “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.”

   “[…] 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.”