Regarding the concept of a game, I would like to describe a less-common model of using 3D content in AR. As a rule, 3D objects are shown on or above some physical surfaces; figuratively speaking, such AR is convex. However, it is interesting to try to create a concave reality by creating an illusion of a hole in a surface. Fiducials create a boundary for such a virtual recess and players have the chance to look inside the rabbit hole. The idea is manifested in the best way when markers are placed on a wall, since the game creates an AR window where some virtual scene is shown. The illusion is pretty smooth because the virtual image has a real-time response to change the player's point of view and position is space of a camera; there are parallax, real time linear perspective distortions, and so on.
Game content is perceived as a truly and natural addition to the real world. It is like having a window into new dimension in your own room. Of course, a special step...