All the dungeons/castles are identified by the name of the nearest feature on the map included with the game.
In the French original, the various castles are called donjon ("dungeon"). In the English localization, instead of being translated as "dungeon", they are designated by another French word, château (French for "castle").
Every castle has a puzzle that consists in taking an item and laying it down in a specific room. When you solve the puzzle, a stanza will be shown. The solution of the final puzzle (for the tenth castle) is hidden in these stanzas.
Whenever you see an item with no obvious use, try the following, in order:
Disarm: it might be trapped (absolutely avoid the Inspect command).
Ask or Read: get information (or flavor text).
Hunt: occasionally, you might get food.
Open or Break: it might be an odd-shaped door.
Take: the last action, because once it is in your inventory, it will be impossible to Ask or Read for information.
French manual
English manual