Fastholds, also known as strongholds, are structures or places that are difficult to capture or invade. Synonyms for fastholds include fortresses, castles, citadels, bastions, redoubts, strongpoints, and bulwarks. These terms are often associated with military structures or positions used to defend against attacks. Some other synonyms for fastholds may include impregnable, impenetrable, and unassailable, which describe a location's ability to withstand attacks and remain secure. Overall, the use of fastholds and its synonyms emphasizes the importance of secure, well-fortified locations in defense and strategic planning.