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Suppose players get to refurbish, repair or straight up build their own fortress. Since it is supposed to be a safe place, a sanctuary, how do I let them protect themselves against magical threats?

In high magic worlds, since both sides are likely to have their own magic users, you can do whatever you want: make the walls stronger, shield against outside magic coming in, erect a giant purple energy dome and so on.

In low magic world it is not much of a concern, since magic is rare and rather weak.

But what can I do if my world falls in the middle?

As I see it, there are three main magical threats:

  1. Straight up destruction spells.
  2. Transportation magic (umbrella for teleports and various magics that would let one get inside e.g. digging through solid earth).
  3. "Social engineering" magics (putting guards to sleep, charming them, disguises, invisibility and so on)

High destructive power of certain spells would probably urge the engineers to build bastion forts rather than traditional medieval castles, in order to mitigate incoming damage. But how can I deal with the other two types?

Just to be clear, I'm talking about military, rather than civil protections. Obviously, high and thick walls are still working pretty well even today, when you are not using them to stop tanks, artillery and bunker-busting missiles.

Anti-magic fields feel a bit like a crutch, rather than a proper solution. And how would magic users do their thing if they own a completely anti-magicked fortress?

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closed as too broad by Miniman, Szega, Pyrotechnical, lithas, Slagmoth Sep 7 '18 at 13:29

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • \$\begingroup\$ Welcome to the site! Are you also interested in spying/scrying/mind-reading magic as a fourth category? \$\endgroup\$ – Joel Harmon Sep 7 '18 at 12:30
  • \$\begingroup\$ Hey! Now that you mention it, I probably would like to restrict these as well. \$\endgroup\$ – Heagon Sep 7 '18 at 12:33
  • \$\begingroup\$ What exactly do you mean by middle-level magic? Why would solutions that work in high magic be inaccessible? If there is enough magic for Offensive Spell A, why is there not enough for Defensive Spell Versus A? \$\endgroup\$ – Szega Sep 7 '18 at 12:41
  • \$\begingroup\$ @Szega I mean that in high magic worlds, both the quantity and the quality of magic users are high, so it is feasible to have in-house mage who maintains defences against enemy mages. If the setting falls closer to the middle, it would mean that there are advanced spellusers, but they would be quite rare to have one affordable for a small castle, yet common enough to have one attack such fortification. \$\endgroup\$ – Heagon Sep 7 '18 at 13:13
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It all depends on what you can expect from an attacking force.

  1. If you can expect your enemy wizards to exist, you can expect them to have Shatter, a spell that completely invalidates defensive structures because a single wizard could plow through several concentric walls. In a world with Shatter and no counter, castles won't exist. You'd end up with bunkers, much like what we have today. The simple way is to ban Shatter from your game, but I like the spell a lot, so I homebrewed a spell called Springstone that makes castle walls harder and resistant to Shatter specifically. (Oh, and for flavor, it lets you hop along the top of them like a trampoline.) Higher tier destruction magic is harder to handle. There's not really a recourse to Meteor Swarm, Otiluke's Freezing Sphere, Reverse Gravity, or Mirage Arcane (lava field). They just wipe out vast zones. Heck, even a single Firebolt can start a fire in your city. All of those spells require line of sight, however, so concentric walls on any strategic castle will be a must to contain the damage. Moats and ditches will also be useful for keeping your enemies away. Most low and mid level spells don't have very long ranges, so a moat could put them out of range. If your players only control a tiny little fortress to rule the locals, you don't need to worry about strategic magic. If they're at a major crossroads between empires, they really might need a city sized bubble shield or even something stronger. Mirage Arcane has a range of Sight and can turn your city into a lava field for ten days, so...
  2. Forbiddance is a spell that blocks magical travel to an area, but Teleport and Gate are really high level. Teleportation Circle is mid level, but can be disabled easily. You may need to home brew a similar spell that blocks flight or perhaps one that alerts you to detect sapping attempts, but it covers the basics. Otherwise, a single mage with Fly could Firebolt your city at night, turning it into a firestorm or dig under your walls with Mold Earth and make them collapse. Depending on the surrounding threats of an individual castle, you may not need to worry about Fly, but Levitate might still give you headaches.
  3. In my campaign, mages caught charming people are executed because of how horribly awry that can go if left to fester. People just aren't trained in it because of how illegal it is and the social norms make people hate enchanters. Detect Magic and See Invisibility should counteract most invisible threats. Sleep also isn't a very powerful spell, even when upcast.. Zone of Truth is also a good catch-all that every courtroom will be enchanted with if at all practicable.
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  • \$\begingroup\$ Why do you say "in a world with Shatter castles won't exist"? A trebuchet deals 8d10 damage, and Shatter deals only 3d8. Castles still exist in a world with trebuchets tho. \$\endgroup\$ – enkryptor Sep 7 '18 at 12:40
  • \$\begingroup\$ I have to agree with @enkryptor on that one... also depending on how the DM defines a "wall" and its construction it could be ruled a structure which is arguably not valid for the spell. Even if the DM allows it, it would definitely be a huge or larger "object" and thus have a damage threshold as detailed on p247 of the DMG. In all likelihood Shatter is not intended as a siege engine. \$\endgroup\$ – Slagmoth Sep 7 '18 at 12:54
  • \$\begingroup\$ War Wolf took (according to Wikipedia), 30 wagons of material, 5 master carpenters, and 49 other laborers three months to complete. Records I've found elsewhere (though can't locate now) say it was able to knock down part of the wall in one hit. Granted, it was the largest ever made, but trebuchets have non-trivial construction times. Also, DMG247 specifically calls castle walls objects. \$\endgroup\$ – Michael Greene Sep 7 '18 at 13:12
  • \$\begingroup\$ Also, trebuchets have longer range \$\endgroup\$ – enkryptor Sep 7 '18 at 13:18
  • \$\begingroup\$ Interesting suggestions. How would you deal with airborn unit attacking a castle? Magical turrets? \$\endgroup\$ – Heagon Sep 7 '18 at 13:18
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Indeed point 1 can be treated as any other damage

Point 2 is harder. You could set up a ritual around the place that disallows teleportation/plane sift and similar spells/effects (maybe with exceptions for those that know the password or something). Preventing tunneling or flyers from attacking you (magical or otherwise) is harder. You could add adamantium foundation or something but mostly let your players be creative.

Point 3. A general anti-magic field would work, or a field keyed to a specific school of magic, or magic items/potions, that the guards wear/drink when on duty.
If you want your own magic user to experiment inside the fortress you can always give him an area where the protection is not present - but this could off course be used against him.

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  • \$\begingroup\$ Welcome to RPG.SE. I made a few edits for format, thanks for joining in. \$\endgroup\$ – KorvinStarmast Sep 7 '18 at 11:51
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Your defenses will need to be multi-layered...

Walls

What the structure is made of obviously impacts how difficult it is to damage - an object's Armor Class and Hit Points are determined by the material. From the chart, Adamantine is clearly the best choice. In many settings, however, it is difficult to work or expensive to acquire in the quantities needed to make an entire structure.

Really big "objects" (like castle walls) also have a damage threshold, any damage less than this value is ignored completely. There's no provided table for what materials should have what threshold. I've always used a combination of the material (often just using the same value as the AC) modified by the construction quality (human stoneworks are around 20-25, while dwarven equivalents would be 30 or higher). Low-level spells like Shatter (unless significantly upcast) can wreck loose objects, but aren't terribly effective against structures1.

Wards

Generally speaking, the best way to fight a spellcaster is with another spell caster. Securing a structure against teleportation, portals, and planar travel is solved by Forbiddance, a 6th level cleric spell.

When the characters have at least ten minutes notice (or as a matter of course) the 6th level bard & wizard spell, Guards & Wards should also be on your list. It creates a whole host of magical effects to protect a structure. Druids can get in on it and help with spells like Mirage Arcane (wizards and bards can learn it to) to conceal the entire area, hiding other defenses like trenchworks and such from prying eyes.

Glyph of Warding (3rd level Bard/Cleric/Wizard) is a classic way to protect areas from physical intrusion. You can either use it for the normal blow-up-in-people's-faces stuff, or add some unique defensive capabilities to your structure. Combine it with well-trained guards who know to drag strangely-acting compatriots to pre-placed Calm Emotions Glyphs (nothing says a spell glyph has to be detrimental) can actually help avoid some of the charm issues - not to mention keeping half-elves on the payroll, too - they're resistant to charms and immune to sleep. It even has password-controlled options, if you wanted to drop it in chokepoints, so people have to pass through it as a matter of course.

Summary

Nothing can make a location completely impenetrable without deep magic, artifact-level stuff (which a medium-magic setting would not regularly have). However, a combination of structure design and smartly placed spell effects can make a stonghold significantly more trouble to access. By the same token, it's a question of whether dealing with that trouble is worth the effort of getting at whatever is in need of protection.

1Otherwise, adventurers would bring caverns down on themselves all the time.

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