About the horse thing on yorktower, players who just go to the fort are able to use their mount as long as they stay out of the tower, as soon as they enter another level it doesn't let them get on their horse. It might have something to do with the tower it self being out side and not inside it's own level like every other fort tower.

But if you want to replicate it just go to the tower with your idevice and get on your horse near the beach area before entering the fort. Or if you have a cloud mount you can just ride over the water into the fort.