A theatron could refer to the place where spectator looked at sacrifices, dances or theatrical performances. There is debate as to whether early Roman theatres had seats. A theatron came to refer specifically to semi-circular, tiered, stone seats for viewing performances.
Source: "The Nature of Hadrian's Theatron at Daphne," Richard H. Chowen. American Journal of Archaeology Vol. 60, No. 3 (Jul., 1956), pp. 275-277.
"Hence creatures that fly high enough above it and those that graze at one side are safe. I saw another opening like it at Hierapolis in Asia, and tested it by means of birds; I also bent over it myself and saw the vapour myself. It is enclosed in a sort of cistern and a theatre had been built over it. It destroys all living things save human beings that have been emasculated. The reason for this I cannot understand; I merely relate what I saw as I saw it and what I heard as I heard it."
Cassius Dio 69.27.3