Seems to be some improvement, but still, it seems the beams are 'floating'. I'm not an architectural expert, but you could try to:
*place columns where two or more 'beams' meet
*make 'beams' near brick walls appear to be inset into walls (if possible)
*make smaller ceiling support structures 16 inches apart attached to larger ceiling structures with weight (wall) support for the larger structures

I like xXxGuitar511's idea. A castle partially composed of a series of caves might be more interesting.
(stalagmites might reach the ceiling one day
stalactites cling tightly to the ceiling)

Textures: I know very little about such things, but you could just try to add some slight variants of existing textures with cracks and / or discoloring.