In the dungeon in my Cold Fens game, there are a pair of secret doors (and some not-so-secret doors) that didn't just open when found.
In a world with See Secrets, Earth Vision, and other seeking type spells, a secret door isn't good enough if it's merely secret. Detection is trivial with the right magic. Nevermind detection with mundane means - guys with very high Perception, supernatural detection powers that say something is behind that otherwise "blank" wall, and so on.
It's not enough to conceal the means of opening them, either. They'll be found just as easily.
Nor is making the doors tough to open. Shape Stone, Lockmaster, and related spells will take a door apart. Mundane means will do for most doors. A door thin enough to be an actual moving door in a small area of space is going to be breakable, eventually. Thick ones have vulnerable hinges, somewhere. Bash, bash, bash - it might take a while, but unless it's very thick, it changed the issue from "how do we open it?" to "how long before it falls?"*
To be a challenge, it needs to be tricky to open. Otherwise they are found trivially and opened quickly.
Here are some ways I've made secret doors, and secure doors, more of a challenge, in both Felltower and the Cold Fens:
- Doors that fade into ethereal (?) insubstantial translucence for those who are wet with unholy water.
- Doors of magic-immune metal set into walls of magic-resistant stone, shot through with anti-magical metal.
- Doors of immense weight and size, with concealed hinges, no handles, and no visible lock, which open only to voice command.
- Doors which only appear at all if the key is brought close to the door.
- Doors which are magically trapped, even to the point of attacking semi-distant casters trying to open them.
- Doors with a remote trigger to open them.
- Doors with a remote lock, which when undone stays unlocked long enough to the get to the door and open it.
- Doors in such tight quarters you can't swing a hammer, battering ram, or other brute force device to get through.
- Secret doors in Low and No Mana Zones.
All of these are generally coupled with thickness, internal or external armoring, concealed hinges (so they can't be attacked with ease). A passive defense like a door will never stand to PCs given unlimited time to work on it. It will slow them down, and cost time and resources - great if you expect to be able to get to your door to chase off intruders or just want to slow them down.
They also reward those who just have the keys, know the secret passcode, know the trick of the unholy water, etc. In other words, the security slows down those not allowed in but doesn't bother the knowledgeable.
None of the above will really stop blowing past these doors with sufficient brute force or sufficient magical power and skill. But these types of special challenges also reward non-brute force solutions and de-trivialize bypassing secret and secure doors.
* The usual counter is wandering monsters and "but we'll be heard." These are not always issues, and a person concealing a door and using it for secure access control can't depend on 1d6 wandering whatevers or alerting the ogre in the next room. I find these aren't really big concerns for PCs, no matter what comes a-wandering if they make too much noise.