Circle is a very strange suspense movie. I categorize it as horror because the people in it are not sure when or why they are dying.
They begin to realize the rules and ways of the Circle, a very sick game they speculate may have been created by aliens to test our human nature.
It's a literal circle in a black room, where if anyone steps off they die. So they are forced to live by the circles rules for about an hour.
The circle kills when someone breaks a rule, or gives themselves up, or another multitude of things.
The circle may have been created to find the "best" people. Watch it and find out!