The one built into an extension of the ninja fort is extremely well-hidden:
The old railway platform baseplates are great for big secret doors, because the big ugly rock pieces can slide on the studless area:
Here are some built into my basic flexible dungeon wall segments:
The one in the middle is a fake secret door, meant to look like it should open even though it does not. More on that later.
Here are closeups of the individual secret doors or panels:
Next we have a secret door that also requires a Lego key to open:
Closeup of the mechanism:
This 'bank vault door' can only be opened with a magnetic staff:
The mechanism is basically just a Lego magnet on sliding plates in a frame
Next we have a wall that becomes a staircase:
Still shots, from the back end:
A smaller one, over a pit:
And the stills:
Using in GameSecret doors are fun to build, but with a regular game, it is too much work to make new ones for each session, and after you use a door technique a few times, players will recognize it. The key to reusing them is to sometimes use the secret door piece as an ordinary wall segment that cannot be opened. Be open with your players that you are doing this. When they are accustomed to seeing the wall segments with secret doors used as regular walls, they will not always assume it can or should be opened.
It also helps to add details to wall segments that look like they might be secret doors, but are not. Once players are accustomed to seeing a lot of things on walls that might or might not be secrets, then it is no longer obvious.
Alternately, you could never place the secret door segment on the map initially, have any normal wall potentially be a secret door, and when they find it, replace the normal wall Lego piece with the secret door piece.