Illusion
Illusions appear throughout The Legend of Zelda Series. They come in the form of doors, walls, floors, or enemies that cannot be seen by the naked eye. However, Link can pass by them, or use an item to see beyond the illusion.
The Legend of Zelda
In The Legend of Zelda, illusions appear both in the Overworld and within the Second Quest dungeons. At the northeast corner of the Overworld, there is a hidden pathway through the northern wall that leads Link to finding a Secret Rupee Location. In the Second Quest, many rooms within dungeons have false walls where Link can actually walk right through them, but the door don't appear visually.
The Adventure of Link
During Link's journey through the Palace on the Sea, the 5th dungeon of the game, Link will come across what looks like a solid wall. However, he can jump right through to progress within the dungeon and acquire the dungeon item, the Flute. Similar false walls can be found throughout the Great Palace
Additionally, there are many enemies in Kasuto and the Valley of Death that Link cannot see, but will still take damage. Link will need to acquire the Cross from the Three-Eye Rock Palace in order to see these enemies.