Hello there!
This is the first time I write here. I've reading about how you plan to connect the astronomy tower to the already existing Hogwarts parts. As soon as I saw the pictures of the new sets yesterday I started thinking about the best way to connect it.
First I'm gonna show a photo of the current state of my Hogwarts Castle. As you see I've added a few extra things, and so far I have been using the Clock Tower as the entry hall where you get from the viaduct. I still have to make the Great Hall tower taller.
I just want to have a good compact Hogwarts silhouette (lack of room), it means that I'm not placing the Clock Tower behind the Great Hall. Instead I think I'm going to remove one of the Clock Tower floors to make it shorter, so the Astronomy Tower would go behind it standing over it. The two mini towers that come with the Astronomy tower would be made taller and go behind the two big towers (a bit like in the real Hogwarts). The removed floor from the clock tower would go, along with the greenhouse, behind the right tower (from our point of view), extending the castle to the right.
I don't know if I made myself clear, english is not my first language.
I hope my idea can be helpful.