mjj - I doubt very much that the makers of the toyline had much (if any) of an idea in their head with regards to the interior layout of the castle. I doubt even that the cartoon creators had much of an idea.
Now, surprisingly (to me at least), there is a huge fan following on the internet for all things He-Man. IF someone has taken the time to do it (I expect that someone has), you could probably find a fan-generated map of the castle somewhere.
I don't have the time to participate, but I would be interested in how your search goes, and if you uncover anything.