Windows are interesting because they connect the inside with th eoutside yet cut the one off from the other. They're a barrier, and one we like to see as hard and fixed, though scientifically glass is a sort of liquid and flows, or at least ancient glass did. But inlike many barriers they're transparent to light and heat. You can see the windows in the movie as a barrier, eg in the trailer between Ennis and the natural world - but the other side of the coin is that windows convey a sense of possibility and show you what's out thee if you want to put the barrier behind you. all this fits so well into the movie.