Zero Tolerance is made up of 40 levels spanning three separate areas: the space warship Europa-1, an abandoned merchant freighter, the heavily fortified central command building for the Planet Defense Corps, and the sub-basement areas of that building. The object of the game is to kill all of the enemies on a level and then proceed to the exit, which is either a staircase or elevator leading down to the next level.
However, nothing prevents the player from heading straight towards the exit without killing all of the enemies. If this is done, the player is simply not given any passwords until the entire area is finished.
Once a character is killed that character will be listed as "deceased," and becomes unavailable for playing. The player can choose from a total of five different characters; once they are all deceased the game is over.
The game supported connecting two Genesis/Mega Drives via a special link cable using the second joypad ports for a multiplayer mode. The cable was originally supposed to be shipped as a pack-in with the game.[1] However this was changed in a last-minute decision and a coupon for ordering a free cable was added instead.