"The people are behind the narrow, tall windows."
Translation:Az emberek a keskeny, magas ablakok mögött vannak.
mögött - behind something. Like az ablak mögött=behind the window.
Hátul is generally in the back. (not compared to some other object) The boy is sitting at the back. A fiú hátul ül.
Not here. A line of paint, a hip of a woman, a small creek can be both but "vékony" is more like "thin". Usually for objects where you want to fit or go through (street, door, window) "keskeny" is used.