Centuries ago, the gnome trapmaster Gryps Wendlehook was tinkering with locks and security. Unable to replicate the intricacy of dwarven designs, Wendlehook decided to incorporate a little magic that the dwarves tended to lack, while including his personal love for puzzles.
The resulting door was essentially a combination lock that employed coloured stones for the combination. Four dull grey stones were set into the door and magically linked to four holes. A combination of four colours was then selected from a choice of six and set as the combination. Anyone wishing to open the door merely had to select the four coloured stones from a choice of six, and place them in the correct order in the available holes. When the correct colour was placed in the correct hole, the corresponding orb on the door would glow the appropriate colour.
Wendlehook recognised that this puzzle would be easily defeated if a determined thief arrived with an appropriate intellect and plenty of time to spare. As such, he included a very obvious trap to act as a deterrent.
The antechamber to any room secured with this lock is required to have a ceiling at least a 50 foot high, adorned with spikes. Every time the code is entered incorrectly, the ceiling drops by an 8 or 10 foot increment until such time as only 10 feet remain. A final incorrect code will see the ceiling drop outright on whoever has tried to bypass the lock.
Inspired by the old board game Mastermind, this door can be employed however the Dungeon Master sees fit.
Both the height of the ceiling and how far it drops for each incorrect guess will heavily impact how many attempts the party get before being ground into a fine paste. This will also rely heavily on how many colours there are to choose from and whether or not repetition will occur in the combination.