Runes of Magic US / AU

Checked this out last night.
You cannot lob over a castle wall, best you can do is hit the top of the wall.
You can lob through the corner turret exits, but the angles only allow you to hit the ramps, cannot hit buildings.

You can, on the newbie gates (not upgraded), fire through the gap between the top of the gate and the bottom of the wall where it passes over the gate. Via this targeting, you can hit the buildings on either side of the gate.

As for the inner keep, you can only hit a person standing just inside the second floor openings, and you are firing blind.

Regarding the video posted by the OP. In a perfect world, there is no way they could do what they were doing. That being said, we are not dealing with a perfect world. When I was trying to lob over the wall and failing (from in front of the castle), I moved to a side gate to fire through the gap. Firing through the gap I could hit the buildings, but my spotter said that she still saw the projectile as if it was coming from my original position, around in front of the castle. So to her it looked as though I was firing through the wall and hitting the building, whereas, from my point of view, I was firing from the side of the castle through the gap in the gate.