I made the MOC's just pretty quickly on an afternoon, so I didn't bother to get reference material or anything; just wanted to give a quick demonstration of the arches idea. 12 width arches are very simple to implement, put two of them together and you have something that is 18 plates wide, add a tile on each side and the whole pattern is 20 plates wide, which just so happens to be 8 studs, which means the pattern (the gothic arch) can be repeated exactly every 8 studs - and everything will fit exactly and hold itself together. The 8 width arches are slightly more trouble, but I've already torn them down, maybe I'll rebuilt a part to show
But I'm sure you guys can figure something out yourselves. Yes, it is pretty solid. In height, the 8 arch is precisely 20 plates, the 12 one is 30 plates tall, so there too you can make them close up perfectly and have it supporting the structure.
You might want to try your luck on the 1x6 arch too, i already found out it's pretty much impossible to make a fairly compact pattern for that one.. perhaps for if you're building big fat fortified towers and you want to have a tiny little window here and there
Whatever you do, don't.