Move your crosshair over this new wall and press V to access the textures, press G to go to a texture and type in 503. Press return and you should have a window. Go back to 2D mode and bring your mouse over the window wall. Press B to make it blockable, so you can't walk through it and press Ctrl-H to make it hittable so you can break it. Note that if you want a perfectly clear unbreakable window, don't mask the wall, but still make it blockable and hittable.
Mirrors are one of the absolute proofs that foresight is best.
Making a Mirror requires you to not just make the mirror Sector, but also a Sector behind the mirror of at least equal size as the area that the mirror will be reflecting . Make the wall blockable and hittable. To make Duke fawn over himself, give the mirror wall a lotag of 252. 17 to make the floor rise. 18 to make both the floor and ceiling lower.