It is possible to be invisible to players on the ground while standing on the top of the keep corner towers and free-cast on targets on the ground.
When jumping the player can even move multiple steps towards the center of the platform and when timed right still cast spells at players on the ground and now even if they should be hit with a CC, they are invisible again and cannot be cast upon.
I can reproduce this issue on request as I cannot attach any pictures here. I will be testing the same issue on Alb and Mid keeps to verify if it is consistent.