Sentence:  Ancient Roman <WSD>amphitheatres</WSD> were mostly oval or circular in plan, with seating tiers that surrounded the central performance area, like a modern open-air stadium. 
Meaning of the word marked with <WSD> tags:  An open, outdoor theatre (which may be a theatre in the round, or have a stage with seating on only one side), especially one from the classical period of ancient Greece or Rome, or a modern venue of similar design.