Apparently there is a path leading right up to the top of the waterfall at the end. It has been blocked off with wooden planks on both sides, but people (like me) still attempt the climb up. It was easy for me to decide to do the climb because people had already broke some planks off so you could just skip underneath it.
The paths up have not been maintained probably since the closure so it's not entirely safe. The paths are quite steep slope paths (they're not stairs), and in some areas near the waterfalls, the water spill over on the paths making it very slippery due to the algae.
Then as you reach to the top, you'd still need to climb through a fence hole by the waterfall. Take note that this is just beside the top of the waterfall so extra care is needed. Once you're in you just need to get to the second floor of that monument up there. It's quite a workout since you start from the palace right to the end and then still do this steep climb.. The view is amazing. Definitely the highlight of my Reggia di Caserta Trip!!!

