[DF] defending a fortress from fliers

[Phantasm]

Obvious solution: Give the fortress its own flying defenders. If your enemy is composed of dragon-rider aerial cavalry, counter with aerial cavalry of your own. If you can't get dragons, use pegasi, gryphons, or hippogriffs.

Plus, there's always towers topped with multiple scorpions, polybolos, and springalds, basically the Low-Tech versions of Anti-Aircraft Artillery ... and if you have gunpowder toss in some hwachas and swivel-guns for good measure.

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And now I have this image in my head of sail-powered carrier ships housing pegasi, hippogriffs, and the occasional SM +1 dragon for projecting air power at sea.

[Michele]

Originally Posted by evileeyore

Invisible Mono-Web Giant Acid-Spitting Sword Spider.

There you go, flyers beware.

Nice!

Another idea: a useful feature of many Crusaders' castles was the bent access. In the case of a world where flying foes are possible, it would be a bent corridor, between the first and the second gate.
In the real world, the L-shaped bent entrance was meant to prevent the use of a significantly sized battering ram against the second gate, as well as to slow down charging attackers.
In a world where attackers hurl missile spells at gates, the casters still need to have a line of sight to the second gate to bring it down with their explosive fireball. With a bent corridor, they won't have this immediately, and behind the spell slits in the wall at the corner there will be the defenders' wizards, ready to engage the attackers.

[scc]

There are two important things to consider when dealing with something like this. The first is that whatever defense the PC's have access to, so does their target but at a higher skill level and likely multiple times over, so if the PC's get the benefit of Reflect Missiles at 25 the fort their attacking has been enchanted 3 times with Utter Dome at 30.

Secondly there's normally a counter to everything, for Reflect Missiles, it a Sunbolt spell.

[Dalin]

Originally Posted by Phantasm

Obvious solution: Give the fortress its own flying defenders. If your enemy is composed of dragon-rider aerial cavalry, counter with aerial cavalry of your own. If you can't get dragons, use pegasi, gryphons, or hippogriffs.

Or swarms of lesser foes, depending on the flavor of the fortress and the type of battle you think will be fun. Doombats, gargoyles, insects, or stirges come to mind.

A fun, non-flying defense could be wall-clinging, nearly invisible giant chameleons who can pluck fliers from the air with their sticky tongues. Maybe they are “blind,” sensing foes through air vibrations (thus bypassing invisibility).

[Gnome]

These are all such great ideas, thank you! I will no doubt use many of them.
The entryway sounds totally unbeatable right now, but of course we know that no GM plan survives contact with the PCs...

[Anthony]

Originally Posted by Gnome

These are all such great ideas, thank you! I will no doubt use many of them.
The entryway sounds totally unbeatable right now, but of course we know that no GM plan survives contact with the PCs...

Broadly speaking there are three solutions to a heavily protected entry: siege, send something ridiculously protected in (the attackers have complete control over timing, so they have options that don't really work in a regular fight), and go around it (through the wall).

The PCs will probably be able to accomplish one of those things, but doing so should slow them down, giving more time for people further away to react and prepare.

[Anaraxes]

Originally Posted by Anthony

three solutions to a heavily protected entry

Historically speaking, I'd add "betrayal" to the list. ("Surprise" is something of a sub-category.)

There's also "screen" -- probably a variant of "siege", but with no real intent to take the place, just mask off the forces inside to render them harmless while the main force proceeds past the fortification. That of course presumes there's something valuable beyond the fortress, as opposed to the fortress being built around the MacGuffin of Ultimate Power.

In a magical world, "go around" includes other options like "go under" (magical tunneling or permeation) as well as interdimensionally "around" (teleport/summons, ethereal/astral travel). That just points up the importance of anti-magical walls (in 3D!) as well as the physical ones.

[Anthony]

I guess we could add bombardment as well, if you don't actually need to get at what's inside. In general the way GURPS Magic works gives a heavy advantage for knowing what the other side has and preparing for it, which makes it very hard for static defenses to survive, so I would assume the point of static defenses is mostly buying time.

[Gnome]

In this particular case the purpose is to take the fortress itself and use it as a rebel base. The rebels wish to avoid massive casualties, so they've called on the (insanely powerful) PCs to take the fortress in their stead.
As I mentioned in the OP, the defenders have the advantage of numbers (and obviously the fact that they're inside the fortress). But they don't have anyone that can go toe to toe with the PCs, so they have to leverage their other advantages.
It's possible the PCs try to simply charge through the front gate, and this may even succeed considering how ridiculously difficult they are to kill. They may try to sneak in through trickery (disguising themselves as enemy soldiers for example). They may try to fly in through a tower or something if that's possible. Siege isn't really an option, but they will probably kill anything that comes out to fight them, and they won't have trouble killing anyone that tries to shoot them through arrow slits or using artillery, simply by shooting back (I figure arrow slits are -7 to shoot through the wrong way, but with skill 30+ this is very doable...).

[Anthony]

Originally Posted by Gnome

In this particular case the purpose is to take the fortress itself and use it as a rebel base.

That is prone to being a bad idea, for the simple reason that, as I noted above, it's a big advantage to know what your opponent has, and presumably the normal owners of the fortress know what its defenses are.