On Castle, go to a little box (can be accessed with the ladder or stairs, at the Castle) now at the largest window, look down and you can see a bit of a roof. Jump on to that.

If you did it right, you'll land safely. Now you'll need to jump onto the closest red pillar.

Then you can just walk onto the roof. You can see everything around you.

You'll fall off a few times trying to get on, but you'll get the hang of it.