On Slanting Passages

Slighlty down-slanting passages that would transport you from one level to the next without you noticing were one of Gary Gygax's favourite deceptions in dungeon design. Going to a lower level meant much more dangerous encounters, doing so unbeknownst not only messed up mapping but could be quite deadly. An insidious trick. 

It was so dangerous, Dwarves even had a special sense to detect this, and players tried ploys like spilling water on the floor, which Gary soon ruled was running into cracks in the floor to block it. Rolling marbles would also work, but I am sure Gary would have ruled the floor was too rough for it.

Sloping Corridor is licensed under CC-BY 4.0. 

The problem with sloping passages is: they do not work. I mean, physically. Even without water or marbles. Gary never did the math on this it seems. By accounts from building standards, a slope of even 1.5% is quite noticable by anyone, normal humans, no dwaven senses needed. Let's give the idea the benefit of the doubt and assume you can get away with 2% slope unnoticed. 

In Greyhawk Castle, each level is likely about 50 feet deep. That means, to go from one level to the next without noticing, you need a corridor that is 2,500 feet or nearly half a mile long. (2% means 2 feet down per 100 feet , or 1 foot down per 50 feet, so to go down 50 feet you need 50 x 50 feet = 2,500 feet of corridor).

The typical size of a level in Castle Greyhawk, along the long edge of the page is about 50 squares. This naturally follows from 4 to 5 lines per inch paper and the page size of a letter page. At 10 feet per square that means 500 feet for the long edge of the map. We have the maps of level 1 and 3 to count it off, too. Level 1 has 50 squares down the page, 500 feet. 

To go down one single level unnoticed by sloping passage you would need to go back and forth the entire length of the level five times. None of the corridors in Gary's maps are that long. Even if they were, it would be highly suspect to walk along a corridor hundreds of feet, and defeat the purpose of unknowingly passing from one level to the other. 

And all of that is assuming you can get away with 2% incline, which you likely cannot. Nevermind going down two levels, that would be a mile long trip. If you used a more realistically unnoticable slope of 1%, we are talking about a mile for a single level.  

Even if you keep 2% and cram the levels tight on top of each other, say 20 feet deep with no high ceilings, you still look at two trips along the entire length of the map, and it would make use pit traps impractical, or at least you need to check if they drop you into a room or corridor on the next level. 

You would need magical distance distortion effects to justify the tactic. At that point, you can just as well use teleporters along indentically looking corridors. Sorry, Gary.

P.S. Someone who also cought this fallacy asked Gary about it on a forum. Gary was obstinate, defending himself thus:

In a dungeon all of the passageways might be inclined up or down. If one can't accept something so petty as an undetected slope in a passageway, how can flying, fire-breathing dragons and all the rest of what makes the genre what is is by justified? A whole lot of suspension of disbelief is mandatory to play FRPGs... [35]

Of course, this is the fallacious "Because dragons!" argument. There is a difference to suspend disbelief for accepting magic that escapes physical logic, and enables dragons to breathe fire and wizards to fly and turn invisible. Yes, there could have magically enchanted corridors to achieve this. But the fundamentals of the world's non-magical stuff are those of ours, and with those, if you are dealing with mundane corridors, this will not work. 

Of course, you could say the same about doors that are always stuck for the PCs, never for the monsters. Or falling that just does linear damage. It is a simplification for game purposes. Just one that is hard to swallow.