Okay.
Many castles have light siege engines in various forms, catapults and such, as a means to damage siege towers and the like.
A Stone Giant's castle might have such things, but they're more likely to be scorpion (a sort of javelin thrower) than catapult. First, because giants are catapults. second, because attacking giants can catch catapult stones. Trebochet type catapults have a very long range, and can fling hordes of darts and similar pointed objects.
Expect to see kegs of flamable stuff, about the size of throwing boulders, to be lit and hurled or launched.
As for location, mountain top fortresses sound neat, but you have to be able to bring in supplies to feed the place, so hilltops and high ground work, but true mountain tops don't.
Use a deep chasm in place of a moat. Harder to "swim", and far better thematically.
Facing a dragon? Consider huge nets made of chain, packed to be loaded into a treb', to entangle a Dragon's wings and bring him down. He flies fast, but is anything but nimble in the air, so hitting him isn't as hopeless as it might seem. He isn't a Nightfury after all .
As long as he stays in the air the giants can't beat him. He can straff and run, and retreat as soon as he starts taking any real damage. If he lands though, and they can close to melee range, he's done. So make the Dragon smart enough to know this.
Also, remember that the Dragon's lair should be a secret. The giants don't know. I mean, if they did then they'd mount an expedition to go after him there, where he's likely to be on the ground, and less likely to run and abandon his treasure.
So make the Dragon sneaky. He always retreats in the same direction, but then drops to a lower altitude , "below radar", and changes direction to the real home. He may even change direction several times, in case he's seen by others.
Also remember that Dragons have Blind Sense as well as Low Light and/or Darkvision. Ranged weapons are only as good as the weilder's eyes. The Dragon knows where the castle is, but the defenders don't know when or from where the Dragon is coming.
I'm talking about the Dragon because it's a possible means to resolve other problems: Party helps with Dragon, good relationships are restored.