What can be seen?
As with most these things it seems rather obvious but takes some brain cycles, so it would be better to think about before hand rather than try to improv. So, can the floor be seen? If not what do the players see? The earth beneath, empty void, a level below? Can the ceiling be seen? If not, do they see the floors above? If multiple levels are invisible does looking across these boundaries just reveal emptiness as far as the party's light carries?
Objects inside a room a most likely invisible (or what's the point?), but what about living things? You probably want the creepy factor of players seeing monsters through walls. So are all living things visible? What about plants? Molds or Fungi? If players kill an orc will the corpse become invisible? If players drop a dagger or 10' pole on the ground will it become invisible? If so, how long will it take? And the invisible objects in the rooms, if players pick them up will they become visible? If so how long will that take?
Invisible exploration is tiring and time consuming
A hallway with two doors requires a glance and a party decision on whether to open one of the doors or proceed down the hall. An invisible hallway requires careful prodding with poles, hands on walls, determining dimensions and that it is, in fact, a hallway and then finding the doors. The second door may never be found. Whole features of the dungeon may not be noticed. If your players can explore 12 rooms in 3 hours, they can probably only get through 4-6 invisible rooms in that time.
Oddly, it takes more attention and brain power to build an image of this invisible imaginary dungeon in a player's mind than a visible one. Eventually, a see-able room will come as a great relief. In my play test I made some rooms visible that I hadn't intended to be, because I saw the fatigue. Also, like in cooking or art, variety is important; if all rooms are invisible, it isn't special when you encounter an invisible room. Because of these lessons learned I have decided that there shouldn't be an invisible dungeon, but invisible rooms in dungeons.
Invisible dungeons require different design
For example, door priority is right out. Decisions on which way to go will result largely from chance; if the party turned left, while feeling along the wall, they will enter the door on the left, even if a treasure chest sits in an alcove to the right.
Patterns and symmetry might help, though even those are hard to discern unless the party is very thorough. But certainly there isn't any need to try to obscure patterns. What I mean is, that in designing my dungeon I made a place that could be livable, with bedrooms, kitchen, dining room and was largely symmetrical. Then I pulled one of my usual tricks which is to distress that normal layout to make it more interesting. I collapsed some ceilings and hallways. But what that left the poor party with was an invisible space that wasn't a natural cavern and wasn't a cleanly cut dungeon. Very confusing and added to the time exploration took.
|The path of the blind on level 2|
So, with all that, I still love the idea of invisible areas in a dungeon, because the invisible dungeon is the dungeon of the four senses. It lets players experience the dungeon more vividly through touch and smell than they would otherwise. But it needs to be used with restraint. Hopefully in the future I will be able to post some invisible dungeon room that can be used individually-- the invisible library, dining hall, and laboratory.