Place tile away from the main dungeon. That is you may want to place it in a corner somewhere out of the way or on an unattainable level eg the level in the DMII example dungeon with the weather control system.
This tile is a non wall tile, that is place it on the floor.
Anything else that is place on the same tile as it, will be considered by the engine to be on the same tile as the party currently is (regardless of where they actually are).
For example, if you put a haze, or flooritem on that tile, then it will always appear in the view. Alternatively (or also!) if you place a trigger on that tile, it will be triggered on every step the party takes; this trigger can then activate any other dungeon mechanics to produce some interesting effects.