Theatre existed even before any play was ever written. Even at that time, theatre was happening somewhere; perhaps a clearing in front of caves or elsewhere in a dark starry night around a burning , more for keeping wild animals at bay than entertaining the crowd gathered around the burning . There was no script, no story and no protagonist.
However, it is clear that theatre happens in a particular and a designated place, which in modern times, consists of an auditorium for the viewers and a stage for the action to happen.
Now the stage can be any place; a piece of land in the field, a street corner, arising on the hill side, a raised mound somewhere, in front of a temple or a ** or a wooden floor raised to some height, theatre is possible as long as there are actors and audience.