It's some sort of weird skylight in the roof that is invisible on the roof map.

The lighting is pretty simple actually. It has three layers.

1. the bright areas are a white overlay layer. This is a little blurred (I used gaussian blur).
2. I duplicated this layer.
3. Next I placed a layer of pure black over the floor (but not over the walls). I selected all the white bits of layer 1 ('alpha to selection') then deleted this off the black layer. Then I reduced the opacity of the black layer to c. 50% or so (can't remember exactly).

And that's it. Very simple.