Questions & Answers about En haut de la colline, on voit la mer.
En haut de means at the top of, high up on, or up at the top of something.
So in en haut de la colline, the idea is that the viewpoint is high on the hill, especially near the top.
A useful contrast:
- en haut = up high / at the top
- en haut de la colline = at the top of the hill
So de is needed because you are specifying the thing whose top you mean.