In that case, you might put a tiny little platform on the hill as a hint that it's part of the puzzle. Cause for some reason using the hill at all makes it SEEM like a completely unintended route (or it did to me at least.) Totally up to you of course, and good luck!
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You can actually get up to it from the ground by firing two shots, so maybe that's the confusion. Now that I think about it, in a video someone made, the puzzle only clicked when they realized there was another ammo machine higher up the tower, so they could afford to use both shells at the bottom. That's another thing I could make more clear to the player.