trench has 6 distinct synonyms spread across 3 of its 9 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.
fortify by surrounding with trenches
“He trenched his military camp”
Synonyms, antonyms and the broader and narrower relations for trench come from Princeton University’s WordNet. Full sources and licences.
cut or carve deeply into
“letters trenched into the stone”
set, plant, or bury in a trench
“trench the fallen soldiers”
dig a trench or trenches
“The National Guardsmen were sent out to trench”
a ditch dug as a fortification having a parapet of the excavated earth