Minecraft wiki entry for Creating_a_challenge_map, associated text: Put numbers along with the game that players have to remember, then add a room to enter these numbers. If they get it right, they can move along. If not, they would fall into a deadly trap, such as lava, cactus, or the void. It is probably a good idea to prevent the players from going back to these numbers somehow, for example, using pistons to create a door that locks behind the players.