You can see the Z-Plan better now - the smaller two towers on either side of the main large one.  There is a staircase behind that stone wall directly in front of you leading up from the entrance to the right of the corner to a small landing and up again to that big opening on the second floor to the left of the corner.

You can go left or right around the building.  You can't go back because of the farm buildings and you cant go forward because there is Nina with her drone taking some aerial views.  Click on the video panel for the 'Clonreher Owl's Eye View.'