One line in the latest chapter (1012) stuck out to me as needing some follow up:
Why would the second floor of the treasure repository connect to the third floor of the castle? The shots inside this connecting hallway have no obvious slant or stairs, so the two floors are probably level with each other. So I spent some time trying to visualise how these structures inside the dome work.
Oda's given us some good overhead maps of the dome interior already (the most recent one just below), but they don't sell the verticality of the place at all.
Given that floor 1 of the castle starts above the Performance Floor and even that has two layers, and is itself elevated above sea level by a long flight of stairs, it was wrinkling my brain trying to work out where on the island and in the dome this treasure repository was meant to start. Failing to effectively illustrate what I was picturing, I searched for a free online 3d modelling software and threw together this blocky thing:
Which I can rotate to visualise all the structures and layers inside the dome at once.
It's obviously not to scale, and you can only get so circular and so intricate in a basic program like the one I used, but it gets the basic layout across well enough, I think. The pale blue represents structures like the two castles and the buildings that branch out from them like fingers on Oda's overhead maps. The structures marked "rooftop access" on top of the main castle are on the roof, above the top of the dome. There's space to move around under Rightbrain Castle and presumably the rest of the Performance Floor, which makes sense, because that area is level with B2 on Oda's cross section castle maps. I think it's weird that the "B1" level of the main castle is aboveground as far as the Performance Floor is concerned, but blame Oda for that, not me.
It's interesting to see how high up the left and right brain castles are above sea level already, considering they're meant to connect to the guard post and the brothel on the outside of the dome. But Oda is one step ahead, showing a lot of stairs and uphill paths around the port as the different groups split off to infiltrate and circle the dome:
The further around the back you travel, the higher you climb through the rocky landscape.
Now, let's look at the back of the 3d model:
Remember, this cuts off the outside of the dome entirely.
Onigashima has to have been built on a steep incline for the ground under the Treasure Repository to start so high above sea level. It could maybe explain the weird placement of the main castle's basement levels though, if those areas really were excavated so the castle could be built. And it's not just my conjecture that the structure here starts high up - look at the steep stairs the Scabbards have to climb when they enter through the back doors:
The Treasure Repository is inside the dome, right behind the gate we see at the top of the stairs here. We get glimpses of the interior courtyard Ashura will fall into while stopping the fake Oden through the open gates during and after the Kanjuro fight, though some of the architecture isn't consistent between that scene and the more recent ones.
Note the lower passage in the bottom left, which presumably cuts through the bedrock and goes into the lower levels of the castle. We can assume this is the path Kanjuro took when he arrived with Momonosuke.
Here's a side view of my visualisation, for maximum clarity:
Oh, and it should be noted that Oda's infoboxes only sometimes refer to the Treasure Repository like it's a separate structure to the main castle. Orochi is said to be lighting fires "inside the castle" but given that the Scabbards met him before leaving the Repository (as stated in the latest chapter), we can assume he's in the back part still. Similarly, Jack and Inu's fight is taking place in the repository as well, but most current "who's where" charts like the one on the first page of the chapter thread don't make that distinction. In fact, given that the repository's floors are counted differently to the main castle's, we'll have to careful not to place these characters on the wrong level in the future.
I hope this helps people get their heads around where everything and everyone is a little better. If this is something people like I might do a version that shows landmarks from the manga, or illustrates where everyone is by my reckoning. But for now, I've stayed up way too late putting this together and need to get some sleep. I'll respond to comments and feedback when I can tomorrow.