My take on fantasy RPG dungeons is that if I pick two rooms at random on your map and there is only one path to get from the one to the other you shouldn't be allowed to call that thang a "dungeon".
I'm not gonna say that the battlemap cottage industry is to blame for the trends I dislike in "dungeon" design because there's A LOT more there, but I do think that for the kind of person who uses battlemaps in their games it would kinda end up being a waste if the dungeon map they're going to use was designed in such a way that, depending on which path(s) they take, the players can end up missing half of the beautifully illustrated setpieces, so there is sort of an incentive for the people who make battlemaps to end up designing them as beautifully rendered and detailed maps of places that are effectively structured like this:
@graham-folger I think this video is a very approachable and beginner-friendly breakdown of some of the techniques frequently used by Jennell Jaquays (legendary adventure designer and probably one of the most influential trans women in the history of gaming) in her dungeon design, and shows an example of applying them to an existing dungeon.
Also, if you´ve ever seen my tutorial post on how to use geomorph tiles as a tool for dungeon map design, you will notice that all of the examples of dungeons I show in that post end up being very non-linear due to the nature of how dungeon geomorphs connect to each other
So I'd say using geomorphs is a very natural way to get nonlinear dungeon maps almost effortlessly.
All the images in that post are very rough sketches, but here's are a little more polished examples of a dungeon maps I designed using my geomorph set:
This is definitely something I seriously struggle with
Trying to balance what OP is describing, and also trying to create a logical and functional structure
I try and temper this by building it in three "layers"
Layer 1: Who built the initial structure and why?
Layer 2: Who repurposed and modified it later on?
Layer 3: Who lives here now, and what do they use the place for now?
That, at least I feel, gets me good results. Lots of varying alternate paths from years of modifications
And yet I still struggle
That's sorta similar to the way i build dungeons too (my approach is usually "populated by at least two factions with conflicting goals, none of which are the original builders") and I think one of the most important tools in your belt for when you're struggling with the "logical and functional structure" part of the process is to remember that a perfectly valid answer to the question of "what purpose was this space built for?" can be "there is no one left who remembers".
Like, the original inhabitants being gone gives you a lot of leeway for not needing to have justification for the existence of every room, and it's cool to embrace how liberating that can be. Like... why are the ruined dining room and the ransacked armory connected by a secret passage that goes through a weird hexagonal room? By all means, you should try to answer that question, but if you struggle with it... well, presumably it served some purpose at some point, but as of right now its original purpose has been lost to time and the new inhabitants either haven't found it, or are using it as a secret treasure room. Of course, you should still try to come up with answers whenever you can, otherwise it will end up feeling like a cop-out to players, but it's perfectly okay and even desirable for a dungeon to have all sorts of weird nooks and crannies that no one can figure out what they were built for.
I think @maximumzombiecreator put it very well in one of her posts
If you're designing using this approach, you don't need an answer for every space. You can instead approach it the same way its new occupants did. Take it for granted that this is the space that exists, how would the new occupants use it? That weird room off to the side that's a pain to access? Well, who knows what it was built for, but it's cold storage now. This weird thoroughfare makes a perfectly good guard checkpoint. This big hole in the floor might have been used for casting spells at some point, but now it's a garbage dump. In this way, it's easy to come up with what rooms are now that doesn't require you to answer what a room was built for. Using this approach, you still want to have good answers for what a room's original purpose was some of the time. If the space just never makes sense, players will stop trying to engage with it logically, and that's a big loss. Plus, using this effect most effectively, you get a lot of value out of knowing the previous purpose of a room. It can be easy for every kitchen to feel similar, but a kitchen that's been built on what used to be a foundry is instantly more interesting and easier to get creative with. But you get to pick and choose the parts of a random layout that look interesting, or that you have an easy time answering for, and make those the parts where the original purpose shines through. And then in the spaces where you're left saying, "What is with this snarl of hallways?" you can just have the answer be, "it's a mystery. Scholars theorize it served a ritual purpose."
