Translation:Do you see the tall tower at the end of that avenue?
Why je in this case, describing where the tower is, but ĉe in the cases when you want to describe what location someone is at? What's the difference?
I think "je" can be substituted for any preposition. It's for when there just isn't a correct preposition you can use, or (like in this case) you're too lazy to think of the correct preposition.