Edit: This question has been reopened!
Medieval castles are mostly designed towards delaying or obstructing ground attackers. The only real threat a castle faces from above, at least from humans, comes from arrows volley fired up towards the battlements and arcing downwards or siege equipment. These threats are negligible if a castle is placed high enough, but for air attacks this isn't necessarily the case. Technology is around the mid-medieval era with the inclusion of fantasy races. It would be best if the castle's fortifications still retained most of their potency against ground threats as originally intended, as ground forces remain the most numerous in world with air units usually reserved in an auxiliary or decisive shock role due to their relative scarcity.
I've decided to group threats into two basic archetypes:
- Airborne Skirmishers: riders on the backs of flying monsters such as a griffon or wyvern, or monsters themselves like gargoyles use ranged attacks to suppress or kill the castle garrison. Carrying stones and dropping them, javelins, archery, incendiary projectiles, etc would fall under this category.
- Melee Assault: melee attackers are inserted onto the defenses via airborne mobility. This would cover riders dropped off onto the battlements or beasts capable of melee attack themselves. Swooping attacks also fall here. Their goal, besides killing garrison troops would be to disrupt ranged defender fire and taking strategic points such as a gatehouse.
Against either threat a standard crenelated castle battlement would have little to no effect. Crenels and merlons only provide horizontal protection. Gatehouses and moats would similarly have no effect. Garrison troops on the wall would have a tough time against skirmishers, and only limited defense against melee attacks. Spears, the most common militia weapon due to ease to use, low cost, and long range aren't so good on walls. They are best in large tight overlapping formations hard to form on thin battlement walkways, and if formed make easy targets for air skirmishers. Bow users are precious with how long it takes to train a proper archer to properly use warbow draw strength. Losing them to melee assaults that get through their arrow fire is suboptimal. Mixing archers and melee garrison forces on the walls might cripple both parties, as spears and bows need free space to maneuver. The battlements on walls that would help them against ground attacks basically ties the garrison's hands against air attack. They need proper defenses to give them a defensive advantage against all attackers.
My first thought was wooden hoardings, or temporary wooden framed extensions built around rampart or parapet battlements. These usually included arrow loops for returning fire on besieging forces, but only horizontally. The roofs were usually solid to provide better arrow protection and sometimes covered in rawhide to counter incendiary attacks. A solid roof, while protecting best against ranged skirmishers also keeps defenders from firing up at them, ceding the sky. They would also just provide good landing spots for melee assaults, who could just land on them safely and rip through to get down to the garrison underneath. Possible solutions include adding wooden spikes on top or opening arrow loops upwards, but both have downsides. Namely increased weight and increased fire chance from wooden spikes, while arrow loops weaken the original purpose of protection against missiles.
My second thought would be to place matching height towers in pairs with their roofs sealed off and gently sloped towards each other. By firing through arrow loops/slits across at each other, the tower pairs could cover each other's roofs versus melee assaults. With the only openings underground or arrow loops, there would be no easy access into the towers from the air. If the arrow slits are above the hoarded battlements, they would also be able to fire down safely on airborne melee assaults trying to go down through the hoardings. This is a rather costly solution, effectively requiring double the flanking towers and obscures wall line of sight towards ground targets at the base of the wall which isn't a big problem with single towers.
The last idea I had would be to give up castles altogether and go full underground bunker. This is a bit extreme, but would force a ground invasion. Naturally this isn't a very pleasing option for matters of prestige, one of the main administrative duties of a castle. It would also be only subpar versus land assaults, being vulnerable to tunneling efforts and flooding in most terrain.
Any ideas or suggestions?