26 April 2017

One-Way Doors, Variable Stairs, and the Accessibility of Sub-Levels

At GenCon 2007 I received a refresher course on one-way doors while co-DMing the Bottle City level of Castle Greyhawk with Rob Kuntz.  This was my first play experience using one-way doors in at least 15 years, and Rob’s take on them was quite a bit different from how I had always pictured them in my head.  In Rob’s view, one-way doors acted as normal doors from the “door side” and once the PCs passed through them, the door closed (automatically if not spiked), and was gone.  From the “wall side” the one-way door was detectable as a secret door, but even then could not be opened with a knock spell.  I had always pictured one-way doors as doors visible from both sides, but only being openable from the one-way side---the other side appeared as a normal door that could not be opened (I may have unconsciously been following the lead of Roger Musson’s “Dungeon Architect” comments on one-way doors here).  I like Rob’s version better, for reasons that’ll become clear soon.

On the Bottle City map, Rob used the now-standard false door symbol to represent one-way doors.  This shared symbol appears in the sample dungeon map in Underworld & Wilderness Adventures on page 4, and denotes a false door on that map.  The now-standard false door symbol is also used as the one-way door symbol in the 1978 monochrome editions of G3 Hall of the Fire Giant King and S1 Tomb of Horrors, and the S1 usage of the symbol is easy to confuse with a false door (a problem even more evident in the 1975 Origins tournament version of Tomb of Horrors).  The symbol for one-way doors standardizes in 1981’s Moldvay Basic rulebook, also used for the 1981 green-cover edition of S1.  One-way doors are specifically mentioned in OD&D:   “Doors which are openable from one side only” (U&WA, page 6)  and “Doors which will open to allow traffic into an area but not out of it” (Greyhawk: Supplement I, page 61).  Appendices G and H of the Dungeon Masters Guide also list “door, one way” and doors and stairwells as sample trap- and trick examples and building blocks (pages 216-217).   

I like Kuntz’s definition of a one-way door---any door that appears like a normal door from one side, but is a wall from the wrong side, with no physical hint that a door exists on the other side.  One-way doors reinforce the utility of iron spikes as standard equipment for PCs, and (if detectable) also provide DMs with leverage against players who constantly search for secret doors---PCs may discover the wrong side of a one-way door, with no way to open it from that side, which will waste time and generate more wandering monsters while they futilely roll d6s…. 

One-way doors can be placed as standard one-way doors or as one-way secret doors, and other dungeon features can easily be defined as one-way corridors, stairwells, chutes, covered pits, etc.  

Variable Stairs*

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
Variable features behave with more uncertainty than reliable, though tricky, dungeon features like one-way doors:  the variable stair can lead up or down, and while it is still trustworthily predictable, the PCs don’t know that, of course---to them the stairs appear very unpredictable, and their maps will be befuddled.  While the change in the state of a variable stairwell from up to down is obvious, it has some subtle implications.  How often does the stairwell change?  What triggers the change?  Can the change be specifically invoked to shift the stair to the direction the PCs desire, and if so, how?  What does the stair look like from the other/up side, when the stairwell is currently going down (and vice-versa, of course)?  The answers to these questions lead to a variety of adventure possibilities for a DM, and potential headaches (and challenges, and perhaps rewards) for the players. 

The key to take away from the U&WA suggestions above, and the idea of variable stairs in general, is uncertainty:  after exploring a dungeon level for awhile, PCs shouldn’t necessarily know with surety that they are still on the same level, and they shouldn’t necessarily be able to rely on their return route being the same as they attempt to exit the dungeon, even if they’ve mapped correctly.  Variable stairs help to drive that uncertainty, in two stages of features:  predictable and unstable.

One of they key differentiators between one-way- and variable dungeon features is their reliability.  That is, a one-way door behaves the same way each time you encounter it:  the door cannot be opened from the wrong side, but when it is opened, it always opens into the same hallway or room (although the PCs cannot return to their original location by simply reversing their direction of travel).  Predictable variable stairs (or other variable features) may lead to different places when encountered at different times:  when first found, variable stair A leads down; when passed on the way back out, stair A leads up instead of down.  That is, stairwell A leads down to level 2 or up to sub-level 1a, but within the scope of those two constraints, stair A behaves otherwise itself.  It is predictable, though variable.  Now, if the DM changes the frequency of stair A’s options, it become a bit more uncertain, and a bit more dangerous:  if stair A leads down to level 2 5 times in 6, and up to sub-level 1a 1 time in 6, that’s going to make level 1a much harder to access (and potentially to leave when the time is right).  It’s also going to make sub-level 1a a place that the PCs can be stranded within, that will require them to pack plenty of iron rations in case the stairwell gods are not with them when they’re trying to leave (or enter) that sub-level….

The second stage of uncertainly for variable stairs is unstable, which is simply a push beyond predictable variation into unpredictable variation.  Variable stairs are variable because they lead to different (though fixed and binary) destinations.  By opening up the destination possibilities for variable stairs, trap doors, hallways, pits, etc., the DM starts to play in the realm of unstable variable stairs (or other dungeon features):  these lead to multiple potential destinations rather than the simple binary options that an up/down stair suggests.  For example, variable stairway B on level 4 key #62 leads: 


Down to level 5 key #4
Down to level 6 key #14
Down to level 7 key #24
Down to level 7 key #44
Down to level 8 key #34
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. 

This new sub-level is territory added to the known and explored regions of the level, but is accessible through a newly-changed dungeon feature (or the newly-discovered property of a rarely-varying feature, if the stair only leads up 1 time in 20, for example).  PCs could learn of the new sub-level’s existence by hearing rumors at the local alehouse, or through finding a map that disagrees with their own, or observing the feature behave differently when employed by monsters or an NPC party, or from legends that “The Fox’s Hole” is only accessible from the SE stair when the moon is full.  Regardless of the method, by employing one-way doors, variable stairs, and the other sundry dungeon feature combinations, DMs can easily insert new levels or sub-levels to expand existing territory beyond the map’s edge, or even to overlap new and existing territory within a level.  That is, with a variable door, an entirely different map may exist beyond the door, a level with features that would conflict with the previously-known level map.

If the new sub-level is self-contained and only accessible from the variable feature, then living creatures will probably be less common on the level unless it has a food supply and/or its inhabitants can create food and water (or don’t require such, like golems, undead, etc.).  Any sub-level isolated by a 1 in 8 or greater chance of not being found becomes a very appropriate challenge for higher-level adventurers, and presumably for placement in the lower dungeon levels.  However, don’t discount the possibility of making such rare sub-levels accessible from one of the upper levels of the dungeon complex, too, since any group of explorers (lower-level or higher-level) are less-likely to find the Hideous Sub-Level of Doom in the first place.  To introduce this concept of variable stairs providing access to sub-levels, a DM should insert some examples that will teach players the ropes of such features:  perhaps some of the sub-levels have multiple means of ingress and egress, and perhaps the variable features are strictly binary, or if they disappear completely they’re only gone 1 time in 2 to 1 time in 4.  Springing such features on players with no experience with this kind of trick can be very frustrating, so building up their confidence by setting expectations with background information in-character, as well as an initially–forgiving play experiences will allow a DM freer reign to turn the heat up on such challenges as the PCs and players grow more experienced. 

In addition to their utility for managing access to sub-levels or to new levels added within a well-known and explored level, one-way doors and variable stairs provide an additional level of variety and challenge within the campaign dungeon environment.  These trick features force players to stay on their toes and also reward careful player mapping---and mapping is perhaps the best way to defeat these devices (along with knock, dimension door, and passwall, of course!).  A word of warning though:  one-way doors are easily susceptible to abuse as channeling devices if over used.  The occasional series of one-way doors that herd PCs toward some special encounter or to a stairwell down four levels is OK, but if one-way doors always force players into yet-another Kobayashi Maru or Catch-22, the features will quickly lose their charm.

I tend to think of one-way doors primarily as trick encounters but they can also act as channeling traps too by forcing PCs into environments that they would not have otherwise explored, or by trapping them in an environment that would not otherwise have willingly chosen to enter, without a known path of return.  Variable doors and stairs are potentially much more dangerous, since they force PCs to wait for the feature to return or reopen (which may take hours or days!), or to explore further in the hope of finding egress and a return to known paths.  If the variable feature opens into a sub-level, however, it’s entirely possible that the feature itself may be the only means of entrance (and exit!) to that sub-level….

One-way doors and variable stairs can help hidden sub-levels to remain hard to find, and hard to return from, and keep your players on their toes as they wonder whether the stairwell that they used to access the Hidden Crypts of Boccob will still be there when they are ready to return to sunlight, fair maidens, and fine ale.  I hope you’ll enjoy adding them to your campaign dungeons! 

Happy Delving!


* I haven’t been able to track down the exact origin for variable stairs as I’ve described them; I’m pretty sure that I read about them somewhere, but it’s possible that I just combined the U&WA descriptions of the trick stairs and added my own twists.  If you find a definitive source, please let me know!

"One-Way Doors, Variable Stairs, and the Accessibility of Sub-Levels" first appeared in Knockspell Magazine #1 (Winter 2009). This version of the article includes the errata published in FKQ#2 that expands and corrects the second paragraph.


  1. I hadn't put much thought into the one-way doors previously. Very interesting. Set 1 of the Dungeon Geomorphs from 1976 has a number of door symbols suggested for use including a one-way door (using the later symbol for a false door) and a false door (a door with a vertical line through it). There is also one I haven't seen elsewhere for a "door, removed" (a door with an x through it), which is further clarified by a note that this is "no door, solid wall" - which I take to mean that there was a door, but it's been bricked over or otherwise walled up.

    1. I'll have to pull out a copy and look at those, thanks Zach!


  2. One of my favorite OSR magazine articles. When I first encountered this circa 2010 it really blew my mind. Well done, Allan.


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