I wrote about the cells here. They're the only inaccessible 'rooms' in the game I've been able to find (the next best thing is staircases to nowhere underneath Priscilla's floor).
https://soulswiki.forumotion.com/t17840-a-collection-of-screenshots-from-the-3d-map-viewer-showing-the-relationships-between-areas-and-some-odditieshttps://imgur.com/a/hlAAL
Basically, there are two "hidden" or unreachable cells that appear to be
real rooms. That is, if you could fly or no-clip to them, there is collision data for the shape of the rooms.
There is not collision/ map data for the two larger cells we see up top in your picture. However, the smaller cell in the middle is real and actually has a drop-down path into one of the cells you can enter. Go into the cell that contains the hollowed Reah, and you can look up and actually see this vertical shaft in-game.
Here is the rear-view, showing the unreachable cell in the top-right:
The second room is right near the cell containing the bonfire (the floating door here
without the wooden planks in front of it):
Here's the rear view, showing that it's not connected to anything:
Here's my guess. Seeing how the only two unreachable cells with real map/ collision data have exits that drop safely into other parts of the level, either or both locations were probably considered as possible invader spawn locations. Some areas, like Sen's Fortess have a lot of places to invade (7 total). While the Duke's Archives only has two, for some reason. If there were more places to invade, these would be perfect spots for that.
Maybe they figured having a spot where an invader could be unreachable would provide too much of a "troll" scenario, or they just didn't want invaders directly invading in the prison after giving it more thought.
If anyone has the PC version with the ability to no-clip around the map, please take a look at these cells!