We don't have synonyms for propylon yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are procyon, propyl and propylic.
(n) :
(architecture, historical) Alternative spelling of propylaeum. [(historical, especially Ancient Greece and Ancient Rome) A vestibule or entrance, (especially) to a temple.]
(n) :
(architecture, historical) Alternative spelling of propylaeum. [(historical, especially Ancient Greece and Ancient Rome) A vestibule or entrance, (especially) to a temple.]