> I wouldn't expect ballista to be able to take down anything but the largest and slowest flyers
The thing is though, that for every single trained gryphon knight or whatever, you could probably have a ballista or several scorpions, easy. (smaller, but still big enough that you want a tripod for it)
The cost of a gryphon egg (if you buy them from the people who steal them from wild nests, is probably very high, on top of that you have the cost of feeding them as they grow up, followed by training, the losses of those that get injured, die from disease, have the wrong temperament etc, etc. Bottom line, a gryphon is retardedly expensive. I think people severely underestimate what an absolute bitch it is to feed something that doesn't eat grass.
Compare that to the cost of building a scorpion/arbalest/ballista. Skilled labor and crew, sure, but probably nothing compared to some kind of gryphon knight.
And when you compare the two, you realize that hey, if the war machine wears down, you can fix it. And if you have skilled craftsmen and materials, you can just keep building them.
Last of all, flying cavalry gets kind of stupid when you consider counter measures.
What is a guy with a bow or a lance on some flying horse going to do when the other army puts a steel cage or big shield in front of their scorpions? Or mounts them on carts with arrow slits?
As soon as you think logically about knights on flying animals you realize that it's actually hella stupid, and probably a terrible investment of time and resources compared to the things that can kill said knights on flying animals.
>Castles are somewhat open to air attack
Sure, castles exactly like the historical ones. But making them safe from flying attacks would be really easy if you put your mind through it. Ropes or chains strung across the courtyard makes it incredibly dangerous for anything with wings to land there, for instance.