It's excellent, but I think you mean the dungeon's gate is destroyed on a roll of 12 *or less*.
Viewing post in The Endless Dungeon of the Mad Mage Morag comments
Good question, my original intent with this rule was to make the amount of times the dungeon could be explored limited, because of the *dank* treasure, it could easily be exploited if it was infinitely explorable. Revisiting this rule, there are probably much better ways to implement it, and i think your suggestion of making it some sort of range is better, especially with the +1 modifier, because then it becomes a probability thing. It should actually be something like...
Roll a d20, on a 17+ the gate crumbles, also add +1 each time they revisit the dungeon.
This would make it so on first visit if they rolled a 17-20 it crumbles, and the next visit 16-20, then 15-20 etc etc. This increase the probability of crumbling each time in a way that actually makes sense.
This could also be flipped so that a LOW roll causes the gate to crumble, which is more inline with how lots of systems treat failure, but i feel like the math would be slightly more complicated, and i always try to make my games easy to grok.
I'll make sure to revise this when i update the pamphlet.