Nice work- in particular the surrounding rocks.
Presuming the roof is missing due to damage, it looks a bit strange that the trussels appear intact. I would also have thought these supports would origionate between the side windows rather than directly above each of them.
I appreciate they may not transfer load all the way down the side walls but I do think they would look nicer positioned this way, might seem more natural.
That being said the wooden beams seem quite lightweight if the are designed to support a heavy slate roof- more suited to a barn,but im no architect. Phaps its because they are a light wood- a dark heavy oak colour might subliminally suggest more strength.
However the large solid structure of the dome definately does look too heavy to just be supported at the edges by the side walls. I'd expect the walls to slope inwards at the 4 corners to bare the weight of the mass of stone.
I would be tempted to place more slate/roof rubble on the ground as well-
and while it's a nice effect, I've always wondered how cracked but otherwize intact slabs lift out of position and rotate as well ( i remember seeing this a lot in UT99 custom maps)
Really sorry to nitpick- it looks really nice. I don't know if you wanted things pointed out, but these points just sprang to mind as soon as i saw it.