How to make an enemy not see you / not attack you from behind a window?

In vanilla you could draw a sector very close to the window and surrounding wall on the back side (but not touching), and then raise it almost to the ceiling and do not give it any side textures. This creates a see-through invisible wall that blocks attacks and sight, but it does not block sound. But that's easily fixed with block sound lines on the window itself. Remember block sound lines must be done in pairs.

In UDMF, things get MUCH simpler. Simply flag the line as block everything. Just for safety, also check the other blocking flags.

I don't have much time at the moment, but take a look at TNT Map 19 sector 405 for a vanilla way to do it. 

Thank you! This solution came to my mind but since texturing the middle sector didn't work, I thought it wouldn't either.