One-way doors are the most-basic element of what I think of as a suite of related dungeon features which limit and/or channel PC movement. Variable stairs are a great example of “upping the ante” from one-way doors, but unlike one-way doors, variable stairs are only hinted at in OD&D. Page 6 of Underworld & Wilderness Adventures mentions:
- Steps which lead to a slanting passage, so the player may actually stay on the same level, descend two levels, or ascend two levels
- Trap steps which lead up a short distance, but then go downwards for at least two levels, with the return passage blocked by bars or a one-way door
- Doors which are openable from one side only, which resist opening from one side, or which appear at random intervals
|1||Down to level 5 key #4|
|2-3||Down to level 6 key #14|
|4-6||Down to level 7 key #24|
|7-9||Down to level 7 key #44|
|10-11||Down to level 8 key #34|
|12||Up to level 4c key #12|
Clearly, stairway B is not nearly as predictable as stairwell A. And we’re not done yet! What if stair B also disappears completely after it is traversed, and doesn’t reappear for 1-6 turns, or hours, or even days!? If the feature is gone, is another area revealed in its absence---that is, with stairwell B not present at all in key #62, is a door (perhaps one-way?) into key #63 now visible that would otherwise be inaccessible while the stairs were present? Worse, what if stair B behaves differently upon ascent vs. descent: the PC will have fought their way through a level 8 encounter, and decided to beat a hasty retreat back to level 4, only to find that stair B now leads elsewhere upon ascent (and perhaps even “ascends” into a deeper level from below!). The possibilities and combinations are practically limitless!
The Accessibility of Dungeon Sub-Levels
Sub-levels are generally thought of as more-remote areas that are offset from the main dungeon’s levels; classic examples include the three levels of WG5 Mordenkainen’s Fantastic Adventure (sub-levels to the main Maure Castle/Castle El Raja Key complex), and the Interdicted Prison of Zuggtmoy and the Elemental Nodes sub-levels in T1-4 The Temple of Elemental Evil. However, using one-way and variable dungeon features, DMs can insert new levels into old, well-trodden paths, by changing an existing door or stair into a variable one.