Castles and DnD

[S'mon]

IMO catapults are indirect fire machines and rely on the target being fairly stationary. I would say that chances of hitting a flying creature with a catapult is about zero. Ballistae on the other hand is probably where I'd go IMC.

Fair enough - although catapults can use grapeshot to devastating effect vs masses of enemy troops, but Celebrim is right that ballistae are the way to go vs dragons & such, a la Reign of Fire.

 

[S'mon]

The problem with spiky roofs is that except against very clumsy fliers, all the spikes simply mean is that its much easier to hang on to or perch on the roof - either for the rogue or a grappling hook. Now, slanted slippery roofs on the other hand...

I was thinking (1) long spikes to deter big beasties + (2) broken pottery, shells, rusty nails etc to damage man-sized climbers - you see this a lot* on the tops of real-life walls, to deter burglars in dangerous areas, at least in UK.

*It may now be illegal to risk injury to burglars though, *sigh*

 

[S'mon]

It's notable that fortresses were still being built in the 19th century, designed with sloped walls to take account of gunpowder weaponry (the German invasion of Belgium in WW2 started with the assault on the fortress Eban Emael, which fell after bitter fighting & heavy losses). Flight definitely changes things though; I think the only real defense vs magical flight is a high-level Greater Dispel Magic zone over the castle's airspace & around its moat, maybe with a narrow non-DM tunnel leading to the gatehouse to avoid ruining potions etc of legitimate visitors. IME non-magical fliers attacking a castle are far rarer than Wizards & Sorcerers so I think DMZ is the way to go for any serious defense.
Basic infantry forts designed vs orc hordes probably don't need these elaborate defenses, though.
I agree w Celebrim's point that a castle should have its own PC-style "rapid response team" with arcane & divine casters, rogue and several Fighter types. If the Lord is 9th level, a team ca 4th-7th level would be about right.

 

[S'mon]

I agree with John Morrow re dungeons - of course you can combine the two, a regular fortress with an extensive dungeon complex beneath.