Xor is a maze game that is played over a total of 15 levels with the option of beginning on any one, though I suggest you cut your teeth on level one.
The aim is to collect every mask that is held within the maze. The number of masks varies from maze to maze, but the total present and total collected are always displayed on the screen.
The maze walls are constructed of brick, and cannot be walked through. Some of the passageways are filled with two other types of material, known as dots and waves. Dots can only be passed through when travelling in a horizontal plane and waves in the vertical. In this way, parts of the maze can be cordoned off until approached from the correct direction.
Maze number one should pose few problems, but the fun really begins on level two. The dots and waves are still there, but they have been joined by fish and chickens.
The fish act in a similar way to the boulders in Repton, falling vertically down when unsupported. The chickens are a different kettle of fish - they "fall" horizontally from right to left. A blow on the head from either of these characters is fatal.
Maze number four, entitled Explosive Mixture, sees the appearance of bombs and bottles of nitro glycerine. The starting point for this level is inside a completely-sealed cell of the maze. You soon learn that dropping fish or chickens on bombs causes them to explode, blowing a hole through the nearest wall.
One very clever feature of the game is the Replay facility which comes into operation when you've lost your second, and final, life - it retraces every move that you made on the current maze at high speed.
It can be used to determine at which point you made a wrong move, or you can use it to revel in the glorious way that your cunning overcame the game's puzzles.
The number and complexity of the puzzles is overwhelming and the fact that there is no time limit allows you to sit back and study the situation.