donjon

Donjon comes from the Old French donjon (the lord's tower, the keep), from the Medieval Latin dominiōnem, from dominus (lord, master). The donjon was the main tower of a castle — the strongest, tallest structure, where the lord lived, stored his treasure, and retreated during siege. It was the highest-status room in the fortress. In French, donjon still means the main tower of a castle.

English borrowed the word twice and split it in two. 'Donjon' retained the architectural meaning: the main tower, the keep. 'Dungeon' shifted meaning downward — from the tower as a whole to its lowest room, the underground cell where prisoners were kept. The lord's tower became the lord's prison. The same word, the same building, opposite ends of the vertical axis.

The Great Donjon at Vincennes, built by Charles V of France in the 1360s, is the tallest medieval fortified tower in Europe at 50 meters. It contained the royal apartments, a chapel, and a treasury. It also contained prison cells. The Marquis de Sade was imprisoned there. The building that held the king and held the prisoner was the same donjon — the meaning split was architectural reality.

English kept 'dungeon' and largely dropped 'donjon.' The underground prison swallowed the tower in English. In French, the opposite happened — donjon still means the main tower, and prison is a separate word. The same Old French word ended up meaning 'the best room' in one language and 'the worst room' in another.