If the keep has a significant amount of magic power at their disposal, you could see:
- A permanent Wind Wall or Fickle winds extending as a Dome over the keep
- Permanent Walls of Force over the top of the keep (also handy for keeping the rain out)
- Anti magic zones surrounding the exteriors of the walls/ covering a "no fly zone"
- Glyphs of warding or other magical traps scattered in the fields surrounding the keep (save for the main trade road)
Hire a priest to unhallow the whole place, and tie a spell effect to the whole place. Dimensional lock is good.
Cast planar binding. Have a few outsiders or elementals around to defend the place if needed.
Try a forbiddance spell, that's a powerful one.
Permanent illusion is good.
Undead are useful to stockpile, if that's your thing.
You'll need to break away from the medieval castle image, and go with a trace italienne model. You'll need to include anti-aircraft emplacements as well...
Uniforms and disguises. If you can get everyone to look the same, scrying and assassination become harder.
Defenses need to be layered. That's the big takeaway.
You have a wall, sure. And guards. But you also need a password to deter shapechangers, and dogs with scent to notice invisible opponents.
If enemies make it into the courtyard, they are still blocked out from the towers and central keep, which will all continue attacking them.
etc.
A wall should be built in honeycomb style with open spaces, simply so you can't passwall through it.
Remember worked stone is very resistant to Rock to Mud. I imagine with magical treatment, it'd be downright impervious.
Anti-fly zones are harder. Bound air elementals would probably be your best bet. Remember to pay them in perfume.
Invisibility purge and alchemical throwing weapons stockpiled should be no-brainers. However, unless this extends a significant distance from the keep, guards aren't going to have any warning of invisible creatures approaching. 3.5 had a watchtower that granted sentries Detect Invisible for just this reason.
Disguises and uniforms thwart scrying, but make infiltration once inside easier. have to strike a balance.
Stillflight fields and Dimensional Interdiction are two very common effects in my home campaign, simply to make it very difficult to pick off easy tactics like this.
==Aelryinth
Buildings can be magically treated, like dungeon walls and doors can. Doing so doubles their hardness and hit points. Magically treating a building costs 5,000 gp for a Large building, 10,000 gp for a Huge building, 20,000 gp for a Gargantuan building, and 40,000 gp for a Colossal building. A spellcaster with the Craft Magic Arms and Armor feat can magically treat buildings.
A ship is a collection of numerous objects. As a result, any ship of Huge size or larger is too big to be affected by this spell.
Source Skull & Shackles Player's Guide
Get a Large stone castle, polymorph it into iron, then magically harden it. Presto, you now have an adamantine castle with 50% more HP than usual- 60 per inch instead of 40. Also from the same source, walls can be magically hardened for the same effect. Get a few permanent walls of force and magically harden them for hardness 60 and 40 hp/CL, which should be at least 520 given the Cl 13 requirement for permanent walls of force.
Well, first there is two important considerations:
1. How common is high-level magic?
In a world where one in five hundred is a caster, and one in five hundred casters is higher level than 5th, things will be very different to a world where the local MageMart is run by a 9th level wizard and every caravan employs a 5th level wizard as a guardian.
2. How much wealth is available?
Magic is expensive. Especially permanent magic.
That said, a few things that I'd consider quite basic:
Walls should not be built in solid rock and left at that. My suggestion for a wall would be more like:
LLLLLLLLLLLLLL
WWWWWWWWWWWWWW
SSSSSSSSSSSSSS
SSSSSSSSSSSSSS
SSSSSSSSSSSSSS
WWWWWWWWWWWWWW
LLLLLLLLLLLLLL
L = Paint heavy in lead. This stops divinations from passing through.
W = Wood. Cheap material and affected by completely different spells than stone.
S = Stone, bound together with mortar containing various non-stone materials to prevent meld with stoning through it.
(of course proportions of thickness are way of above; the layer of paint is extremely thin)
There should be a gap between the wood and stone layers, to stop things like Passwall and slow down Disintegration.
I think regardless of setting, magic is rare enough that it's incredibly rare that mages _alone_ would take a castle; I see it more as them being an important support for an army.
Due to this, I think the most important defenses vs flying enemeis are to have loads of guards when you suspect enemies might come. Guards with decent ranged attacks, watching the sky with ranged actions to shoot at any mages or weird stuff happening. Preferably, there should be a few low-level mages with them too, readying stuff like Glitterdust. It takes a quite high level (and specifically built) caster to pull off Fly + Invisibility + Silent attack spells efficiently, at least if the attack spell is supposed to be good.
In addition, I think every castle should have a few air-mounted elite troops as well. And the perfect mount is the Dire Bat. A combat trained one costs only about 450 gp, though I assume they'll have bat handlers there doing the training. Having a dozen 3rd-4th level warriors on that and you have a great air force; the double move speed of 80 ft. combined with the blindsight of 40 ft and the +12 perception allows quite a lot in terms of detection, and the rider can act independently.
In addition to this, if the castle can get a hold of a bunch, some pseudodragons are even better at detection; with their int10, blindsense 60ft, darkvision and lowlight vision, double move speed of 120ft and natural stealth of +19.
I think the fact that castles that looks like medieval castles exist, is proof that magic-users are rare enough that they aren't considered a big threat. In a world where magic users on a regular basis where part of an army and high level enough to do crazy stuff, castles would not look like medieval castles.
They would be bunkers. They'd have domes, probably be smaller, and be spread out. They'd have a lot of magical defenses and traps used as single-use items to set off when there is an emergency.
Well, the answer can obviously widely vary depending on if we're talking about the most powerful nation's strongest citadel, or if we're talking about some motte and bailey guarding an unimportant border.
In general, the main thing would be to know that your average castle would be defended by, frankly average people. Getting anything much beyond a level 2 or 3 spell is probably not going to happen when the local mages are Adepts. Because of this, I'd suggest that the physical construction of the castle is the most important part, and that the spells I'd recommend using are simple, cheap spells. I mean, if you happen to have a level 12 caster on hand to fortify your castle with, cool. But casters are supposed to be rare, good ones nearly impossible to find, so with that in mind, here's my suggestions.
First of all, would be a different wall design. As mentioned, if someone was using a spell like passwall, your castle becomes less than useful pretty quick. So the idea of primary, secondary, tertiary defenses would probably be very key in this setting, so that even if a powerful spell is used to get past the outer walls, there's still a lot of castle left to fight through. The walls should also have a space in the middle as mentioned above by Aelryinth. I'd personally suggest leaving that spot open to the top of the walls to create a murder-hole scenario for anyone who tries to breach the walls that way, and have these wall-pairs be connected by short bridges and only be a few feet apart. Give someone just enough room to hang themselves, basically.
A roof, while silly to laud as a key part of your defense would also be extremely important if you could manage it. Flew over the castle? That's nice, now climb off the roof and get down here to the door so you can actually go in. :P This would only be viable if we're looking more at a underground castle or at least one built into a cliff-face Helm's Deep style though, otherwise the amount of support structure work you'd have to get in place to get a roof suspended over an entire castle would be less than cost efficient to say the least.
Something we can take from modern tech as an alternative to this would be much cheaper, but only effective at night. But since that's when most people would be trying to infiltrate castles with fly spells, it may well be enough: Tall poles and wire. Nowadays we have to hang those funny orange balls in telephone/electric lines to keep helicopters from getting tangled in them, but if the point is for tricky mages to get tangled in them? Well ignore the orange ball part and move on, at the very least, you're taking precious flying time away from the caster, which may turn into a nasty fall if they're not careful. And if they can't see well in the dark? Well the guards on duty may well become aware of the intruder as the cries of surprise for running into a wire net would likely cause, leading to alarms being raised, and a whole lot of arrows flooding the sky.
As for teleportation spells, there's not too many good ways to stop that that doesn't involve high level spells, but there's one way you can fudge it a little with architecture. Build the whole castle as two facing. One way to defend from external threats, the other to defend from internal ones. Obviously if you do this, the enemy will probably try to use the defenses against you themselves, which is why the defenders need an ace in the hole to keep this from being detrimental. Namely, a series of secret passages and such for the defenders to move around the choke points and kill zones if their attacker gets a hold of them. All this adds up to making it so that even if a mage gets a small army teleported into the citadel of the castle, the defenders can just lock the thing down from the outside and start using the citadel as a trap rather than just a wall to hide behind. Obviously, since knowledge of the secret passages makes this whole plan go wrong, you'd have to be very picky with hiring practices, and attackers would likely look for insiders to help them.
Now the first spell I'd suggest for magical defense? Stupidly simple though it may be, Continual Flame. If you're working with a Motte and Bailey or otherwise wooden castle, fire's obviously a problem that this solves, but more importantly, regardless of what your castle is built out of, this prevents people from blinding your defenses with darkness spells. When you're one of 50 guys trying to defend a castle from an entire army, being blinded and not know what's going on is pretty much a death sentence to the defense of the castle, so a magical, permanent, fireless light source that won't take up precious storeroom space in the event of a siege? Priceless.
As far as wands go, I'd get Hold Portal spells, great for keeping the battering ram at the gates from getting in too fast while you brace the door or keep throwing crap at the rammers, etc.
If there's a crypt in the castle, definitely try to make sure it's a sanctified crypt, so that if you're being attacked by a necromancer, you can make sure that any losses on your part don't come back to eat you. Or I suppose if you're the necromancer, desecrate that crypt so you've got a way to restock your numbers.
Nap Stack is also a good option, level 3 so towards to top of my self-imposed cap, but the defenders are going to want to be awake for as long and often as possible, so this spell is really good for that.
Wands of Create Food and Water would make a very compact, easy storage way to keep emergency rations for a prolonged siege, so in a magical setting those would probably have a few set aside. Probably some wands of Purify Food and Drink too to help with rationing.
Spike Growth is also a good one to slow down a attacking group so that you can pick them off with ranged fire. And since that one runs on the hours/level basis, definitely has some staying power for a siege.
The spell Eagle Eye would have excellent use for the garrison commander. After all, it's basically the strategy game POV spell, so it's hard to go wrong with that in this scenario.
The last thing I'd recommend would be wands or potions of Expeditious Retreat, so that the defenders can quickly fall back to a second line of defenses or respond to an unexpected breach by an invisible attacker, teleporter, what-have-you.
My main point comes down to, most castles, despite being impressive fortifications are running on a cheap budget once they're built. And awesome mages are hard to come by. So most of these defenses should be either mundane, or low level spells, and the fort's defenders and physical walls are probably going to be what's really relied on to defend it, since permanent high level spells are probably not in the budget/not available.
Duderlybob: It goes both ways thouh. If you're hardpressed to get access to more than 3rd level spells, then it stands to reason that an army that has 5th level spells easily accessible is already incredibly overpowering to you. Though I think if the country has a 10 or so level wizard in employment, we can assume long-term improvements has been made by that wizard since she can get there pretty easily through teleport (or wind walk in the case of a cleric).
Other than that, you have a lot of great ideas.
I'd add that a wand (or a few scrolls) of Entangle is great, if you can get them. Planting thorny bushes around the castle makes it even more fantastic. It has a _huge_ area, covering 5000 square ft., and a range of 440ft at level one. Sure, the save is low, but when you're stopping hundreds of 1st level warriors, even a low save will get enough to slow down the whole warband.
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A lot of good points above here. I agree with the ideas about layered defenses/defense in depth. Some more points:
* Embrace diversity. A wizard may be able to answer any single question, but can he answer all of them at once? Use guards with different strengths and weaknesses. You need a different spell to get past a construct than you need to get past a trained human.
* Decide beforehand what you castle is for. Is it to defend the realm against powerful enemies? I suggest that you plan an offensive strategy, where you actively seek out those enemies and destroy them, before they come for you. The goal of the castle is to get 8 hours of rest so you can refresh powers before taking the offensive again, not to hold out a siege. Against powerful casters, defending a castle is in the long run hopeless.
But one major historical role of castles isn't to defend against other (would-be) kings. Kings have cannons that will eventually wear down your castle.
The role I'm talking about is controlling the local population. If you have a castle that's pretty much impregnable to a mob of 1-5 level wannabe adventurers and commoners with pitchforks, that'll let you dominate a region. You can seek out dissidents and smack them down, but they can't suddenly turn on you and drive you out, because you have thick walls.
But make sure to have scouts and spies so you learn about nosy PC-types in advance. And don't sit in your castle waiting for them to come to you; be proactive.
* Cover. Make sure the countryside around your castle is devoid of cover, so people can't come close without you shooting arrows and spells at them. Meanwhile, you have parapets giving you cover and higher ground. It should also give your siege engines more range than theirs.
* Underground. If there's no way to approach the castle unseen above-ground, enemies may try Earth Glide or even plain tunneling. Hide some nasty surprises in the cellars. A few tame Shadows or Wraiths will terrify a druid. (Easier for a lich/vampire BBEG.) Also, have metal grating in your walls just like reinforced concrete.
* The human element. Historically, many succesful attacks on castles were the result of the attacker entering the castle by surprise or deception. If the attacked had to actually siege it was much much harder to take the castle. So make sure your guards are protected against mind-control and illusion (diversity - use both living and construct/undead/plant guards, and animals with scent). Also make sure they've got sufficient Perception and Sense Motive.
* Teleportation - I don't have much experience with this, but the Teleport Trap spell seems like it could be a lot of fun. Also, use of long-term illusions might be used to trick people into making teleport errors.
* Scrying/Gather Info - information security is vital; if enemies can't learn about your layered defenses beforehand, it gets harder for them to prepare solutions for all of them. This is important, but I don't have a good plan on what to do about it yet.
Ilja: It does indeed, and that's a good point, but generally, if you were fighting on equal terms, it'd probably turn into a pitched field battle as opposed to a siege. So yeah, whatever nation's out there probably does have a level 10 Wizard on payroll somewhere, I'm just assuming that he's probably not enchanting every fort and castle out there since he's a rare commodity and would probably be focused on defending the key points like a capital city, or going out and warring it up with the neighbors. People really only attacked castles when they thought they had an overwhelming advantage since castles were such tough nuts to crack in their hayday. So I actually did assume that the attackers would probably be rocking a stronger magical arm since they're the ones on the offense.
I do like the lead paint idea for the walls though, it's a brilliant little touch on a rarely thought of rule that really helps in a big but simple way. I may suggest putting the lead on the inside of the wall though, would help protect that portion of the defenses from siege weapons and such, so more like.
SSSS
SSSS
LLLL
WWWW
LLLL
SSSS
SSSS
The mortar is a cool bit too, yet another thing that's a simple, reasonably cheap way of thwarting magic.