hmmmm now I'm stuck LOL

Looking good. You can carve the arched ceilings out of foam.

Foam would work, though it might be difficult to get a uniform curve throughout. When I built my portico, I cut curved supports and then glued flexible veneer to make the curved ceiling. http://www.otterine.com/blog/blog1.php/the-portico-part-two and http://www.otterine.com/blog/blog1.php/the-portico-part-three You might be able to adapt that idea to your room.

The problem I see is the window to ceiling height. With the supports, curved wood layer and paperclay, you might not end up with a very pronounced curve. But, it might be just enough to give you the impression you want. Maybe bend a piece of cardboard in there to see first if you even like it.

ahhhh.... good thought Brae....and Christina.....

The opening on the left is actually the door.... this kit is designed so the Tudor addition has different floor levels from the tower.... at first I thought I'd change that, but decided to keep it instead, because it's unusual.... I also thought about removing the door opening all together at one point... but I am going for a bit more realistic on this one.... a bit... LOL

If I can't figure out a curved ceiling... my other thought is just paperclay (no stones) then stretch beams side to side instead of front to back... and cover the beams with paperclay and carve them to look like stone.... I'm gonna try the cardboard visual too....This is my project today