Learn English – Differences between “fortification nouns”

This is one of the cases where English has a bunch of words that mean very nearly the same thing because of its many overlaid mass borrowings from its neighbors, and in this case also because of the rich and ancient tradition of warfare in Europe. If you pick the wrong word, native speakers will still understand, but they may get slightly the wrong impression. Don't worry about it too much.

I'm going to explain the words in a slightly different order than you listed them.

• A fortification is any structure whose primary function is to aid a military defense. It is most often used for structures whose only function is defense, such as city walls, trenches, tank barriers, etc. Fortifications might not be permanent structures; the trenches of World War I were dug as needed and filled in when no longer useful. There's a related verb: you fortify something when you prepare to defend it (not necessarily by erecting fortifications). This term is in current usage.

• A fort is a building or group of buildings designed to serve as a base for an army. It also serves as a fortification in the above sense; it may for instance have artillery emplacements, walls designed to resist cannonfire, etc. This term is also current, but is fading out of the language in favor of "[military] base", partly because traditional fortifications are less and less useful in modern warfare. Forts often have names: "Fort Pitt", for instance, is the name of a fort (built in the 1730s and now almost entirely gone) near where I live.

• Fortress, castle, citadel, and stronghold are all roughly synonymous with fort. There are, however, differences of nuance:

  • A castle is or was the home of a feudal lord, as well as being a fort. Therefore, people still build forts, but nobody builds castles anymore, except as a deliberate anachronism, e.g. Cinderella Castle at Disney World. The word is sometimes applied to particularly fancy estates to suggest that the person who lives/lived there is acting like a feudal lord, e.g. Hearst Castle; in this case the building(s) in question need not be defensible.
  • Fortress and citadel are older synonyms for fort and castle respectively. They suggest that the building so described is larger, more impressive, and harder to attack than a simple fort or castle. (Citadel originally referred to a fortress "commanding a city, which it serves both to protect and to keep in subjection" [OED], but modern usage does not reflect this.)
  • Stronghold can technically be used for any well-defended place whether or not built for the purpose, but actual usage is basically the same as fortress.

• Finally, a palace is the home of a monarch or other high-ranking feudal lord (a duke or a prince, perhaps). It is not necessarily a castle, and in fact often deliberately not a defensible fort (consider Versailles). By extension it is sometimes used to refer to any large, ostentatious, expensive dwelling. This word is in current usage.

Etymologically, fort, fortress, and fortification are all from the French root fort = [place of] strength; palace and citadel are also from Norman French palais, citadelle; castle was borrowed directly from Latin castellum and then reinforced by Norman French castel (which became château in Modern French); and stronghold is Old English, a composition of strong + hold.

Other related words include:

• keep the central, most-defensible part of a castle or fort.
• hold any fortified dwelling or place of refuge. This noun is basically never used in Modern English except by writers of fantasy.
• arsenal a fort whose primary purpose is to store weapons, and/or a naval dockyard.
• manor [house] the unfortified home of a minor feudal lord.
• fastness Old English equivalent of fortress, also basically extinct in Modern English; the OED doesn't trace the etymology back far enough for me to confirm my suspicion that this sense of fast is the Germanic equivalent of fort.
• garrison originally equivalent to fort, now refers the body of troops assigned there.
• bunker modern military jargon, a mostly-underground fortification designed to protect people from aerial bombing.