Castles in a D&D/Fantasy setting

I've been working on developing a castle or fortress that would be able to defend against common threats in fantasy world. Right now I'm trying to make sure I haven't forgotten anything that might change the architecture of the castle. I have divided threats into 3 categories: flyers, land based attacks, and threats from underground; I'm ignoring outright magic and spells right now because that can change with what game system is in play and the prevalence of magic. I'm looking for potential fantastic threats from the world itself such as monsters or new tactics of armies based on common fantasy tropes.

For Flyers there are soldiers on flying devices and mounts which can take part in aerial assaults and then there are just flying monsters like dragons, rocs, and griffons. Land based attacks can be divided into giants who can act as early medieval cannons; large monsters such as dinosaurs, juggernauts or other creatures that can just flatten walls; soldiers riding giant lizards or other creatures that can climb walls; and treants or the use of magic that can cause plats to grow fast enought to crush stone walls. threats from beneath can be divided into burrowing creatures like the purple worm, carrion crawlers, fyrsnaca, or the desert leviathan; goblinoids or other humanoids that live underground and might just dig tunnels into the middle of the fort; and creatures like the earthquake beetle or magic that can cause earthquakes or upset the foundation of the fortress.

Is there any fantastical threat I'm forgetting? I'm looking for general categories of threats, I don't need to know every type of humanoid that might dig tunnels, just if there is a way that a standard castle would prove to be ineffectual in a standard fantasy setting.

Well, if you're concerned about monsters, you might also want to consider the kind of monster that suborns the defenders: harpies, dryads, vampires...anything with a charm power. You probably also have the category of "monsters that don't care about physical reality": any monster with teleporting, incorporeality, or that moves across planes.
"monsters in disguise" might be an issue too: everything from shapechanged metallic dragons to werewolves, oni, and hags.

This concept has been thoroughly investigated in several old issues of Dragon magazine...the paper one from TSR. I can probably dig the issue up if people are interested.

Magical construction materials can take care of some of the magical attack methods: wyvern blood stops teleport-through attempts, &c.
A capital city might have a 'magic tower' which just happens to hold the gadget which is a focus for the no-fly-bubble over the city. (FR's Waterdeep has something similar; I used it in my Rise of Tiamat campaign to explain why dragons weren't doing the London Blitz thing whenever the anti-Cult of the Dragon Council met.)
There should be a way to fortify stronghold walls against Stone to Mud spells.

If these don't already exist in your favorite game rules, make something up and insert it as lore for your campaign. Of course the PCs can do research, if/when the subject comes up.

I'd also encourage you to search for threads on EnWorld. The topic has been repeatedly discussed at very great length, and the response you may receive to this thread does not necessarily reflect the mania the fanbase has for this topic - just the fact they've done it to death.

In a previous ENWorld thread someone mentioned using magical banners on castles, keeps, in large units. The banners had various magical protections baked into them that prevented a tightly packed of phalanxes from being cooked by a fireball for example or teleportation into a castle. Banner are incredibly important, symbolically and practically, much like Roman Legion Eagles at the height of the Empire.

I had an adventure where the PCs had to capture the castle's banner to make it more vulnerable to magical attacks. Kind of like taking down the shield in StarWars.

pogre said:

In a previous ENWorld thread someone mentioned using magical banners on castles, keeps, in large units. The banners had various magical protections baked into them that prevented a tightly packed of phalanxes from being cooked by a fireball for example or teleportation into a castle. Banner are incredibly important, symbolically and practically, much like Roman Legion Eagles at the height of the Empire.

I had an adventure where the PCs had to capture the castle's banner to make it more vulnerable to magical attacks. Kind of like taking down the shield in StarWars.

Click to expand...

Then it becomes a game of "Capture the Flag!"

Thanks for all the replies!

I've looked at some of the discussions that have already taken place here and in other places, but they don't really provide what I'm looking for. I'm looking at physical threats that can be counteracted by physical defenses. Outright magic or things that break the laws of physics are another matter entirely. Things that can go ethereal or teleport at will is not something I had thought of, but I don't see a way of defending against such abilities without magic.

Let's take an example of what I am looking at: Giants. Most versions of giants describe that they can throw large boulders and gives rules for how much damage that can do, etc. Taking those rules at face value you might equate giants to catapults or other siege engines, which weren't commonly used to knock down walls. Based on that you might assume the stone walls of a castle would be sufficient. But let's compare giants to normal humans for a moment and extrapolate from there. Any decent baseball pitcher can throw a ball at 90+ mph accurately at a target. Now if you give him a stone instead of a baseball, the throw might not be quite as fast, but it will do more damage on impact. Suppose a Giant trained in throwing object were given a lead ball, similar to a cannonball, and consider that damage he could inflict. what if the giant were simply using a sling appropriate to his size with lead bullets? wouldn't he then be comparable to some of the early cannons? so we might see a development in fortresses similar to what happened in the real world where high stone walls were replaced by low wide earth and timber walls.

That's just an example of what I'm looking at. If the only way to defend against a creature, such as a ghost(who is unaffected by physical barriers), is through the use of magic then it wouldn't really affect the physical fortification. For the moment I'm taking for granted that all the necessary magical defenses are always used, but if a threat can be neutralized though physical defenses then that will often prove more economical and castles might look very different. I'm just not sure if there are some physical threats I'm missing out on.

Thomas Bowman said:

Then it becomes a game of "Capture the Flag!"

Click to expand...

Nothing particularly wrong with that. It is not the only approach, though. How castles change depends on: how much world building appeals to you. Official D&D material generally ignores it, so it often includes typical high Middle Ages era castles with no elements geared towards magical infiltration, dragons and/or fliers in general.

In general that works if you declare that attacks by fliers and battalions of mages are rare enough to worry about and the PC's are a very rare exception.

If you want otherwise then a lot depends on your world assumptions, for instance, in my opinion if fliers are rare but mages are not then most fortification works would take the form of landscaping. Build artificial hills and thick hedgerows to block lines of sight and create strong point, with judicious use of block houses to funnel attackers in to prepared kill zones. Sort of like the German Siegfried Line defences in WWII.

One of the nice things about 5e is the existence of spells that aid in the strengthening of fortresses and can with some effort be something one would imagine be within the resources of powerful kingdoms.

The best fantasy "castles" are Dwarven underground fortresses. Just like bunkers in the modern age of war.

ardoughter said:

If you want otherwise then a lot depends on your world assumptions, for instance, in my opinion if fliers are rare but mages are not then most fortification works would take the form of landscaping. Build artificial hills and thick hedgerows to block lines of sight and create strong point, with judicious use of block houses to funnel attackers in to prepared kill zones. Sort of like the German Siegfried Line defences in WWII.

Click to expand...

If you want to go whole-hog, find a book about the design and construction of France's Maginot Line, before WW2. Even Hitler, who liked to demonstrate that nobody else could influence his decisions, decided to go around it rather than through it.

Other stuff that is not part of "building a castle" but helps defend it:
- carrier pigeons to deliver messages back and forth to the King's Army (which we hope is riding forth to break the siege of Castle Strongpoint).
- A few trained hawks to capture the enemy's pigeons
- Speak With Animals and ask a squirrel to scamper about looking at things
- as above but the mouse is sent to gnaw through catapult ropes
- "Pony Express" network through prairie and semi-desert terrain, does UPS's job
- Stone walls along farm fields can be built shoulder-high on purpose, so archer squads can travel the land, pick a good spot to let off a flight of arrows at an advancing enemy column, and use the wall(s) as cover of their retreat. Note that cavalry will have to be trained in Equestrian to get over the walls in their effort to outflank the archers
- Those fields have the most sturdy walls facing the roads
- Those roads zigzag at about 2/3 of longbow range, so an archer squad can conduct a fighting retreat sniping all the way
- One- or two-story hardpoints that contain a week's worth of provisions for a squad of men, placed at longbow range from each other, along the roads towards the castle. The point is to delay the enemy and give him a bloody nose (and maybe demoralize him because he's already suffered casualties and spent time before even REACHING the castle). I should also mention the secret trapdoor in the floor to allow the garrison an escape.