Yep, you’re correct so far. If you block some things off, you can get to something like this: 
I put walls to the up and left of the boss, because I don’t think there’s any advantage to making the boss room any larger than it needs to be. The other 2 walls are required to be there.
After this point there are probably a couple ways to make progress, but one way is to notice that the right key is not allowed to see the boss door, or the circle next to it. Therefore you can’t approach the first door from the bottom…