I was revising along and hit a point where I had quite casually done so. Which started me thinking.
Because, of course, this was no ordinary door. Within the realms themselves, Euclidean geometry reigns, even if you have doors in odd places, like tree trunks. The doors are the lynchpins of the non-Euclidean aspects.
Which is why a mere avalanche, caused by an annoyed dragon, might not be enough to destroy one.
You can destroy realms, to be sure, which would have some drastic effects on the doors attached, but it was a pretty dramatic effect all around, and it took some heavy duty stuff.
Which is why I concluded that the dragon won't be able to destroy it.
Pin it open with rocks by filling up the doorway and the corridor behind, yes. Destroy, no.