"The Secret Door is disguised to resemble a normal wall, thus blending seamlessly with its surroundings."
The Secret Door is a type of door available in Dungeon Keeper 2 with the special ability to disguise itself as reinforced wall to the enemy.
To your enemies, the Secret Door appears as another stretch of wall, while your creatures can see and use it for what it really is. This property makes Secret Doors very useful for hiding important rooms and areas from your opponent, such as Portals and your Dungeon Heart, as well as large Treasuries or Gem Seams.
The downside of the Secret Door is if the enemy sees your creatures using it, they will know the truth and are free to attack it. Secret Doors can also be revealed by trap-savvy Creatures, such as the Troll or Thief. You can detect enemy Secret Doors by trying to select the tile for digging. If you cannot keep a tile selected, try digging out the tile next to it to destroy the door.
Once the Secret Door is located, it offers poor defence (about the same as a Barricade) and will quickly crumble.
Interestingly, the Dungeon Keeper 2 Manual states the Secret Door as having high resilience, despite its low health, likely due to its ability to conceal itself.