3D Monster Maze is an adventure game developed from an original idea by J. K. Greye and programmed by Malcolm Evans. It was originally released in 1982 for the Sinclair ZX81 and required the use of the ZX81's 16 KB memory expansion. It is one of the first 3D games developed for home computers, and also one of the first games to contain many of the elements which today would be considered as survival horror.
The game uses a 16-by-18 square maze which is procedurally generated each playthrough. At the start of the game, a Tyrannosaurus rex is randomly inserted into the maze. The goal is to escape the maze without T. rex catching you. When the player begins moving, T. rex starts to pursue them. As the game progresses, T. rex may either become more active (as the player gets closer) or fall asleep (if the player moves far enough away). If the T. rex makes visual contact with the player, it will relentlessly pursue them until it loses sight of them.
The level of T. rex alertness is communicated to the player via several lines of text at the bottom of the screen:
REX LIES IN WAIT: T. rex is stationary in the maze and does not know where the player is.
HE IS HUNTING FOR YOU: T. rex is moving, but does not know where the player is.
FOOTSTEPS APPROACHING: T. rex hears the player moving towards it.
REX HAS SEEN YOU: T. rex has spotted the player and begun pursuit.
RUN, HE IS BEHIND/BESIDE YOU: T. rex is one move away from capturing the player.
by default, the player is able to move faster than T. rex, and therefore can usually outrun the monster unless trapped in a dead end.
Points can be gained whenever T. rex is actively searching for the player, as well as for each step in the maze the player makes. More points are awarded for escaping the maze and loading a new maze. If the player manages to escape the maze, they are told they have been "sentenced to roam the maze forever" and can either continue playing in another maze or can appeal. Appeals have a 50% chance of being rejected, which means being sent back to the maze. A successful appeal ends the game and resets the ZX81.