I was going through some old gaming materials, part of my 2e collect, and came across the remains of this little gem: Dungeons of Mystery.
The product, a skinny boxset, typical of many other similar 2e releases, also included fold-up cardboard rooms that you could use to build out a dungeon lair. The set did come with a variety of room sizes, and a few other flourishes, like stairs and an alter set up for key encounters. Also there were "doors" that you could attach to the fold-up walls anywhere to show where the door would be for a given room.
I remember building out some, or maybe even all, of the "rooms", but these were really nothing more than 1 in high squares in various dimensions, and all with the same bland stone pattern. To build out a dungeon, you were supposed to use paperclips to attach rooms together. So, for instance, you would attach a 20x20 fold-up with a 10x30 fold-up, and add the door piece to show there was an opening between the two. For me, the biggest issues were how bland the walls looked, leaving all the rooms generic and uninspiring, and the fact that none of the rooms held their true shape. None of the corners were 90 degrees and all of the walls bowed out in the middle. In the end I chucked the cardboard but saved the rest and I'm glad I did.
The book that comes with the product is a great find. It covers a lot of area for DMs to think about with regard to their own dungeon creations, such as what was the structure originally, who's there now, how the ecology of the lair inhabitants "level out". Each topic in the book gave me plenty to think about and new ideas that I want to incorporate in my next dungeon build.
The book runs about 64 pages, with just under half of the book being a system-agnostic essay on dungeons. The remainder of the book is filled out with instructions on the fold-ups, and three scenarios using the fold-ups and maps. It should be noted that all the adventures are second edition, and if you have a good working knowledge of 2e, you can easily convert them to 5e.
Lastly, the boxset also comes with what I call the Pinwheel of Death. Aligning any of the monsters by matching color (green, blue, yellow) gives you any instant snapshot of who in in your dungeon, from the boss monsters, the minions, and the vermin. You use this as a way of quickly generating a dungeon on the fly, or to just give you an idea or two in the planning stages.
So, in finale, Dungeons of Mystery, turns out to be a great product, and one that I'm moving from my 2e back pile to my ongoing reference/inspiration books on my 5e bookshelf. Check it out and you may end up doing the same thing.