Minecraft wiki entry for Creating_a_challenge_map, associated text: Make piston doors with multiple keys. One idea is to make an End Portal Frame with a redstone comparator next to it, that must be activated with a hidden eye of ender. Another option is to make a combination lock. You could also make one where the player must rename a stick to the correct name (hidden on a secret sign!) and put it in a hopper chest, powering a comparator for the door.