Wow...first that is a awesome castle you've got going there. Especially how it isn't just straight grey walls and towers and stuff, it has colour and more interest. Awesome work...can't wait for the next stage.
As for the arrow slit - I came up with the idea myself. When I was building the keep I wanted a unique sort of window idea for it, but I couldn't come up with anything original that I hadn't seen done already. I also knew I really wanted the 'cross' look to it, I personally like it much better. Then I was finally getting pretty annoyed with trying stuff that wasn't working, then I had a few arches and headlight bricks from other attempts there and then suddenly realized those might have a chance of working. And then as a bonus the headlight bricks have the little bottom part to make the horizontal opening a bit thinner. Then it took me forever to get it to work in the wall and fit, but it worked out eventually. (Thanks to the guys in C-C chat too!)
I am really not surprised someone else came up with it before me though, although it would have been cool if I was.
Now I wrote far too much there when this is a thread about your MOC, I apologize, but I just wanted to set that straight. Can't wait for the next stage of the castle, keep it up!