I've been playing with water physics for some time now while building my adventure map, and I found a very simple device that can open doors. In certain situations, its better to use than the standard way to open doors. I'll explain by first mentioning keys. Keys work in a way that the key itself is made in a way that when slid into a lock, if the pins are satisfied, the door is unlockable. The same concept can apply in minecraft using water and holes.
Set up a water system in which you must refresh the block next to it in order for the water to flow into the other direction (abusing a glitch to use water flow as power). Next make a path for the water to flow, as long as needed. Next create holes of whatever depth you desire. Then build a system around it in which players must place falling blocks into holes that drop down into the holes you made. If the right number of blocks are placed, and the water refreshes and flows, it will flow all the way through. Place a redstone torch at the end, connected to redstone, which is connected to a not gate, which is connected to a door. The water will hit the redstone torch, which will turn the redstone off, which will turn the not gate on, opening the door.
Some images of my testing model:
One good thing about this system is that, when its built with pure obsidian, it cannot be breached if the hallway is long enough. A downside AND a perk is that, if you mess up once, you can't reset it, because you have to manually reset the water. This system is perfect for Adventure maps for optional treasure or optional paths. The reason being that there are multiple inputs, and it can give a punishment for the wrong selection plus the fact that it is singleplayer and multiple players can't fudge the system up. There's many clever ways one can punish a player by making a bad input using levers and then pressing a button to see if it worked, but there are several differences from this system and those:
#1 There is only one punishment: You are unable to proceed
#2 The system is ridiculously simple to the point that one does not need serious knowledge of logic gates to build it, you only need to know how to glitch water, know that water destroys certain blocks and not gates.
#3 The system cannot be read blankly unless studied for long periods of time, which requires several tests, but if your system allows it to be ruined without the door opening if TNT is placed into the holes, then there is no real way they can check it, as they can't reset their work, they can only measure the height once by completely filling the holes.
#4 Even if the system is turned on and sand/gravel is not placed in the system and somebody tries to unlock the system, they would have a ridiculously hard time doing so, because the parameters are not known. In a lever system, for example, if there are 4 levers, there are only 16 ways you can have the system set up to unlock the door. Holes cannot be this easily read, they can be any depth.
The cons, though, are obvious: If players can place and destroy blocks at will, then they can seriously ruin the system, be it TNT or whatever. Also, in multiplayer games, the system can be easily clogged, because other players can lock out the system and you are unable to reach it to reset it, the only thing you can do is throw TNT into the holes (if its built with obsidian obviously) and set the sand heights to the correct input. Also, if someone puts the sand to reset the water first, it can be eventually broken into, it just takes a long time based on how deep the holes are (using the concept that the player cannot tell where the hole ends and the hallway begins). This can be thwarted by making all of it but the hallway obsidian, encasing that part of the hallway with dirt, so if TNT is used, it just completely ruins their chances of getting the correct input ever. Then again, if TNT is so prominent, they would blow the door up. I also suggest this in general, as a player can read the depth by dropping a lot of sand (this takes forever if the system is made to be deep enough, so its not that big of an issue), then finding the depth pattern and testing each depth until the lock opens.
Also, i've tested the system, and it works pretty nicely. Said before, but I suggest the system for people who don't want to be seriously redstone heavy. Its a very simple device.
Thats true for any system. A lot of adventure maps etc have rules for that sort of thing. Used in general, using a lever system, or a pressure plate system, or whatever, the door is still the same and you can just blow it up or punch through it. What you're considering would be useful for any system, but this system is focused on opening the door, not the state the door is in. I just assume that they'll respect the door itself and use the system to open it.
Also, I edited it as it has occurred to me that someone CAN measure the depth of the holes by merely throwing sand/gravel down, then blowing it up, taking the difference in depth into account and then constantly taking different heights until the lock goes. I'm recommending the concept of dirt inside an obsidian cage to ensure that this will not work.