WordNet records no synonym and no antonym for roundel. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
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.
English form of rondeau having three triplets with a refrain after the first and third
round piece of armor plate that protects the armpit
(heraldry) a charge in the shape of a filled circle
“a hollow roundel”
Synonyms, antonyms and the broader and narrower relations for roundel come from Princeton University’s WordNet. Full sources and licences.