If one looks at a picture of an ancient castle of medieval days, it can be seen that one of the towers, usually the central one, dominates all the others and the countryside around.
It was also the strongest part of the castle, the part where the defenders, if forced back, might withstand a long siege or regain their strength.
For this reason, this tower was often called a “keep” in England, meaning a place that could be held or “kept” against attack. But in France, and sometimes in England, the tower had its name from its dominant position and was known as a donjon.
Donjon, in Old French, was a corruption of a Medieval. Latin word, dominio, which meant dominion or mastery; in England, the spelling was usually dungeon.
Moreover, because of its impregnability, the dungeon or “keep” served as a lodgment for prisoners, who were kept in the dank, gloomy vaults beneath the massive structure. To them and to all who feared such cheerless confinement, dungeon meant a dark, underground cell, rock-walled and comfortless.
It is this meaning that has survived, and the archaic donjon is now used to designate the tower above.