Origins
The word 'dungeon' has undergone one of the most spatially dramatic semantic shifts in English: from the highest point of a castle to its lowest. The same word that once meant a lord's tower now means an underground cell, and the story of how it got there involves a journey through Latin lordship, French fortification, and English imprisonment.
The Latin root is 'dominus' (lord, master), from 'domus' (house, household). 'Dominus' was the standard Latin word for the master of a household, and in the medieval period it became the title for lords, nobles, and eventually God himself ('Dominus' in liturgical Latin). The same root produced 'domain' (a lord's territory), 'dominate' (to lord over), 'domestic' (pertaining to the household), 'dome' (originally a house, then a cathedral, then a rounded roof), and the Spanish title 'Don.'
In Vulgar Latin, a derivative *domniōnem (the lord's property or the lord's place) developed, and in Old French this became 'donjon' — the main tower of a castle, the lord's personal stronghold. The donjon was the heart of medieval castle architecture: the tallest, thickest-walled structure, designed as the last refuge if the outer defenses fell. The lord's private quarters were typically in the upper levels of the donjon, with storage, kitchens, and guardrooms below.
Spelling and Pronunciation
Important prisoners — nobles captured for ransom, political rivals, royal hostages — were held in the donjon because it was the most secure part of the castle. They were typically confined in chambers on the lower floors or in the basement level, which was often below ground. Over time, in English usage, 'dungeon' (the English spelling of 'donjon,' adapted to English phonology) came to be associated specifically with these underground or subterranean prison spaces rather than with the tower as a whole.
By the fourteenth century, English had split the word into two: 'donjon' (borrowed directly from French to mean the castle keep, the architectural structure) and 'dungeon' (the English phonological adaptation, meaning specifically an underground prison cell). French, meanwhile, preserved only the original meaning: 'donjon' in modern French still means 'castle keep,' and the French word for dungeon is 'cachot' (from 'cacher,' to hide) or 'oubliette' (from 'oublier,' to forget — a cell into which a prisoner was thrown and forgotten).
The modern pop-cultural meaning of 'dungeon' — particularly in the context of 'Dungeons & Dragons' and video games — extends the underground sense further, making a dungeon any subterranean labyrinth filled with dangers. This usage has so thoroughly dominated popular imagination that the word's original meaning (a lord's tower) is now virtually unknown outside of architectural history.
Legacy
The semantic inversion is telling. The same word that once signified power and lordship — the dominus in his domus, the lord in his tower — now signifies powerlessness and confinement. The lord looked down from the donjon; the prisoner looks up from the dungeon. The word traveled from the top of the castle to the bottom, and its meaning traveled with it.