Sentence:  We had previously visited the building prepared for the assemblage, which was a vast pentagonal semicircular <WSD>amphitheatre</WSD> of painted wood and canvas work, ... 
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.