Donjon is one of the words that is quite ancient and is not used very often nowadays. It is mostly associated with medieval times and refers to a large and imposing tower or castle keep. In case you ever need a synonym for it, there are a few options available. You could use strongholds, fortresses, castles, citadels, or even towers as alternatives to donjons. These words all refer to imposing structures of defense and security. They signify the strength and imposing nature of the structures and evoke a sense of awe and admiration. These synonyms can come in handy when describing historic or fantastical settings where castles and fortifications are prominent elements.