This isn't truly a classic game but it is a very unusual take on a classic arcade style game. Everything seems normal, for a few seconds anyway, and then it gets interesting.

Just like a classic arcade game the instructions are very simple:
Use the arrow keys to move.
Use the space bar to fire.
Shoot the ships and do not let them run into you.

HINTS: If you just want to experience the game and see what it does then move to one side or the other at the beginning of the game and then just stay there and hold down the space bar to fire.
This is a game you have to play just to see how interesting it is and to see how such a simple change to a game can increase the challenge so much.
This one is sure to challenge your spatial orientation abilities.