fortification has 1 distinct synonym spread across 1 of its 3 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
defensive structure consisting of walls or mounds built around a stronghold to strengthen it
the art or science of strengthening defenses
the addition of an ingredient for the purpose of enrichment (as the addition of alcohol to wine or the addition of vitamins to food)
Synonyms, antonyms and the broader and narrower relations for fortification come from Princeton University’s WordNet. Full sources and licences.