Coffins as doors/transports is definitely one of those From things that gets stuck in your brain. And yeah in retrospect, with more space, I probably would add some more clarity around the gold and some specific rules so if the players’ could try and work out a way to get the treasure.
The Annex Dungeon term I just made up, I couldn’t find any preexisting term for it when I looked even though I can off hand think of a handful of previous ones and am sure there are plenty more.
Thanks for checking it out.