Bastillion is a fairly obscure word, but its synonyms are more widely used and well-known. Some alternatives for this word include fortress, stronghold, citadel, castle, keep, and tower. These words all have a similar connotation of a heavily fortified, impenetrable structure that is designed for defense or security. Other related terms might include rampart, battlement, bulwark, redoubt, or embrasure. These are all words that are used to describe different aspects or elements of fortified structures, such as walls, parapets, or defensive features. Overall, there are many synonyms for bastillion that can help to convey a sense of strength, protection, or impenetrability.