From the research I've done, what we think of castles these days (these things) were basically only the keep. Nowadays typically they are the only part of a castle that remains, but back in the day they were the innermost defense basically, and afaik did not really have any living quarters in them.
Here's a basic rundown of a castle layout on wikipedia.

The place where daimyou's residence would typically be is a palace wholly separate from the keep/castle tower, and it could be either in the innermost circle of the castle (honmaru) as the keep, or in the ni-no-maru or the second circle. (Here is one example.)

Nagoya castle.
Nijou castle (has a floor plan of the ni-no-maru palace. More on Nijou castle.

Basically, the tl;dr of it is that the Nijou castle's ni-no-maru palace is pretty much the only such palace still standing so finding any other floorplans than that one I linked is going to be nearly impossible imo.
But it might also be helpful to look into Japanese architecture in general or into shoin zukuri?

If you're a visual person, you might also want to consider watching a historical drama? Nobunaga Concerto and Gou - Himetachi no Sengoku are some of the ones I've enjoyed that are set roughly in the same time period as Inuyasha.

Edit:
Apparently Hikone Castle and Kumamoto Castle, like Nagoya Castle linked above, have reconstructed the palaces. So might look into those for further inspiration if need be. The Kumamoto Castle website show the whole castle area. The Nagoya castle website has a whole section of the palace including video and floorplan so definitely worth checking out!

Edit two:

Basic overview of a castle layout.
Nagoya castle layout.

Hi everyone,

I'm looking for resources on Japanese castles - specifically interior design/layout. For my story, Inevitability, I would like to provide descriptions of where guests, samurai, and the Lord/Lady would reside. I've watched several tours on YouTube and I've (attempted) to look at some pictures, but nothing is overly comprehensive or explanatory.

Ideally, I'm looking for information on the floor plan(s) of a Japanese castle.

Any information or guidance would be greatly appreciated!
-FM

There's a show on the NHK where they go into details about specific castles. I think it's called Castle Quest. That might be helpful? You can find episodes on their website I think. (Or by searching for the show on Google).

One more thing, I'll shut up after this I promise.

This is from Stephen Turnbull's book Japanese Castles 1540-1640:

Thank you so much! I've taken a look at the Kumamoto and Nagoya Castles, but I haven't looked at the Nijou Castle yet!

I'll also have to check out those historical dramas as well!

Oh my goodness, this is exactly what I was looking for!! Thank you so much!

Chie wrote:
One more thing, I'll shut up after this I promise.

This is from Stephen Turnbull's book Japanese Castles 1540-1640:

Happy to help!