If you want to go to Purple Mountain Observatory , you have to conquer Purple Mountain first :)
I mean the Purple Mountain Observatory is located on one top of Purple Mountain you if you want to go there you need to climb up to the top and if you still have energy then you can spend 15 Yuan for a ticket to go Observatory, and it`s a great place to know some knowledge about Astronomy and you can have a whole view of  Purple Mountain when you at one terrace inside of Observatory.

Overall, it will be a good trip and you also build up your body by hiking.  Ohhh, one more thing, i highly recommend to go there in the morning, because the fresh air when wake you up .