Not exactly an observation deck. It looks more like an arrow tower from world of warcraft. The view is, as it were, extensive, but not striking in beauty. In the same Kalemegdan from the fortress it is more interesting. But the park itself is good. It's worth a visit. You just shouldn't expect much from the observation deck.
I left my legs until I reached this site :D Well, it's a cool look, yes. The river is a small river, the roofs of houses in Nezin, and that's it. No, of course it's beautiful, but it wasn't worth such a hike with leaving your feet. So if you are not a fan of stupidly walking for 15-20 minutes along a broken-down curb in only one direction, stay at home :D