Check out the video below for ALL the super builds in the game, this includes their location and what it does and also where possible the intereactions. This video also includes super builds that are in the special assignment stages.
First, go to the old abandoned castle that is not far from the Farm. Continue to the very top of the castle. Then when you are on the very top left pillar of the castle (where you can see a boarded up door), look to the right hand side at the other pillar and look down and you will see an icon. Then just jump onto it. (It is recommended you use the Farmer MCain disguise to glide to it.)