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The item Lyre of Building is a rare magic item from Tasha's Cauldron of Everything:

Lyre of Building

Wondrous item, rare (requires attunement by a bard)

While holding this lyre, you can cast Mending as an action. You can also play the lyre as a reaction when an object or a structure you can see within 300 feet of you takes damage, causing it to be immune to that damage and any further damage of the same type until the start of your next turn.

In addition, you can play the lyre as an action to cast Fabricate, Move Earth, Passwall, or Summon Construct, and that spell can't be cast from it again until the next dawn.

(Emphasis Mine)

It seems that an army of bards could make a castle wall indestructible, by dedicating their reactions to blocking damage. This, paired with a dome-shaped castle wall and one of the permanent planar-travel-blocking spells, should be a perfect way to exclude intruders. Food could be supplied by Create Food and Drink, so the defenders could survive indefinitely under siege. Adding more bards would solve the problem of enemies using multiple damage types in one round.

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    \$\begingroup\$ Do you see any problem with supplying an army of bards with a rare magic item each? \$\endgroup\$
    – Kirt
    Mar 9, 2023 at 19:01
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    \$\begingroup\$ If an organization has sufficient resources to pull this off, at least in any of the ‘standard’ comparatively low-magic settings usually associated with 5e, then it’s likely that they do not need to be operating on a defensive footing to begin with (if you seriously have that many bards, you have one of the most powerful propaganda engines and spy networks in the world at your disposal, and even in the pseudo-medieval setting of 5e, winning the information war means you probably prevent the actual war). \$\endgroup\$ Mar 10, 2023 at 2:42
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    \$\begingroup\$ @AustinHemmelgarn: Getting the bards isn't all that hard (all you need is folks with a single level of bard to qualify to attune the Lyres). Sure, leveled characters with PC classes are rare-ish, but if you control a small country, you can probably set up a bardic academy for peasants with even the slightest spark of talent, and get a platoon's worth in a few years or less. Getting enough rare magic items to equip your platoon of bards is the real hurdle; even if you make them yourself, you'd need some moderately leveled artificers or other spellcasters and a lot of money. \$\endgroup\$ Mar 10, 2023 at 16:21
  • \$\begingroup\$ @ShadowRanger Bards specifically are more a case of most people in most settings not having absolutely nothing better to do all day than learning to be a bard. Just like wizards and to a lesser extent clerics and paladins, they are functionally academics in a medieval setting, and such people were generally a comparative rarity because only nobles had the spare time to spend studying things like that. So having that many bards means the kingdom in question has a serious labor surplus (because they need enough labor to support that many people not supporting themselves). \$\endgroup\$ Mar 10, 2023 at 17:39
  • \$\begingroup\$ It is maybe worth noting that in real-life medieval sieges tunnelling was also a popular method, sometimes they'd go completely tonto and there'd be multiple levels of tunnelling going on, with defenders trying to undermine the tunnels of the attackers trying to undermine the walls and so on. So you better have your castle built on solid rock. \$\endgroup\$
    – biziclop
    Mar 14, 2023 at 11:19

5 Answers 5

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It seems so, until the enemy diversifies their siege.

If your enemy is only coming at the wall with a single damage type, such as bludgeoning from catapults, it seems your defense works just fine. But what if they have catapults and ballistae? Since a section of wall can only be under the effect of one Lyre of Building at a given time (by the "Combining Game Effects" rule1), it is still susceptible to piercing damage while it is immune to bludgeoning.

And if the enemy is well equipped with arcane capabilities, they could conceivably lay siege with thunder or force damage as well. Your strategy will only slow down a diverse arsenal, it cannot make your wall immune to everything at the same time.


1 "Different game features can affect a target at the same time. But when two or more game features have the same name, only the effects of one of them —the most potent one— apply while the durations of the effects overlap. For example, if a target is ignited by a fire elemental’s Fire Form trait, the ongoing fire damage doesn’t increase if the burning target is subjected to that trait again. Game features include spells, class features, feats, racial traits, monster abilities, and magic items." (DMG p. 252)

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    \$\begingroup\$ Not to mention non-damaging spells like Transmute Rock, where POOF a 40 foot cube of the wall simply turns to mud and flows away. \$\endgroup\$
    – aaron9eee
    Mar 9, 2023 at 18:55
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    \$\begingroup\$ Any truly diversified siege of course includes sonic attacks. \$\endgroup\$
    – Kirt
    Mar 9, 2023 at 21:32
  • \$\begingroup\$ @aaron9eee if you found a way to make the wall magical, it would be immune to Transmute Rock. Though I don't know if there's a spell that could make the stone magical, so you'd have to work with the DM to find some way of making the wall inherently magic. The only RAW way I know would be if the castle were created by the Instant Fortress magic item (which obviously means the walls are magical). Bonus: it then automatically also has immunity to nonmagical non-siege weapons and resistance to everything else, plus you can't open the door with Knock or similar magic. \$\endgroup\$ Mar 9, 2023 at 21:42
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    \$\begingroup\$ Whether the "Combining Game Effects" rule actually works the way you describe here depends on the rules interpretation. You could argue that for every attack against the wall the most potent effect is chosen, the most potent effect being the one preventing the most damage. According to this rules interpretation you could make the wall immune to everything except for effects dealing multiple damage types at once... \$\endgroup\$
    – fabian
    Mar 12, 2023 at 8:43
  • \$\begingroup\$ @fabian That interpretation sounds a lot like benefitting from multiple instances of the effect at the same time, so it seems pretty clearly to be the wrong interpretation. If your understanding of the “effects don’t stack with themselves” rule is that an effect stacks with itself, you should probably reconsider that position. \$\endgroup\$ Mar 13, 2023 at 13:38
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An object or structure you can see

One clear limitation to the 'invulnerable wall' is that the army of bards has to be able to see the structure they are protecting. Obscuring the wall with a darkness spell will negate their ability to grant damage immunity against even a single damage type. If the defender is employing an army of bards, presumably the attacker has a larger-sized army capable of casting darkness (if they don't significantly outnumber the defenders, they have no business attacking a fortified position to begin with). If the defender can counter darkness cast on the wall itself, then cast the spell on the siege engine shots before they are fired and they will bring their own darkness with them. You wouldn't even have to do it in flight.

One could argue that darkness wouldn't cover the entire fortified structure - even if darkness covered the exterior of the wall, the part being damaged, it might not extend to the interior part, the part the bards could see. Depending on your definition of 'one structure' the bards might be able to look at a part of the wall far away from where the damage was occurring. However, I think it is best here to refer to the DMG section on Statistics for Objects (pp. 256, 257), where we are told that for Huge and Gargantuan objects:

If you track hit points for the object, divide it into Large or smaller sections, and track each section's hit points separately

Thus, if the siege weapons are damaging the wall, they are actually damaging it in 10' square (size Large) sections. These sections can easily be covered by a single darkness spell, preventing the bards from seeing the part that is being damaged and thus from preventing that damage. In fact, if the wall is more than ten feet thick, a bard on the inside would not be able to prevent damage to the outside, even in the absence of a darkness spell. Ultimately this will be a DM's call, but I find such a ruling to support verisimilitude far more than saying any bard with a view to any part of the curtain wall could prevent damage to any other part, even on the other side of the castle, simply because they are 'the same wall'.

Other spells that interfere with vision can also be productively employed. The attacking army can use things like fog cloud with their low-level slots (on a calm day, this has an even larger area of effect than darkness and in addition can't be seen through by bards with truesight) and with their high-level slots cast things like sunburst (which would be quite effective at blinding any bards it did not kill outright).

Or, on a windy day in which the defending bards all have some ability to see in magical darkness, the attackers can employ sappers to simply tunnel under the wall, which was a RW strategy for dealing with fortifications and is even more effective if you have summoned xorn or rock-affecting spells.

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  • \$\begingroup\$ Darkness can be countered by truesight, but Fog Cloud is a physical obstruction, not lack of light or illusion, so it would work even for an army of bards with truesight. \$\endgroup\$ Mar 10, 2023 at 1:39
  • \$\begingroup\$ If you're inside the wall, you can see the wall, even if the enemy casts darkness on the outside of the wall. \$\endgroup\$
    – Adeptus
    Mar 10, 2023 at 2:53
  • \$\begingroup\$ @Adeptus depending on the thickness - 15 radius is 30 feet across. Also depends on whether the entire wall is something, or the wall is separated into compartments as suggested by DMG \$\endgroup\$
    – Kirt
    Mar 10, 2023 at 3:51
  • \$\begingroup\$ Obscuring the wall also obscures it for the attacker too. In terms of real-world considerations this would degrade the ability of attackers to target or correct their fire. By the rules, this should definitely affect spells requiring sight of the target; I'm not sure how the obscuration rules map onto more conventional siege engines or things like giant-thrown boulders though. \$\endgroup\$
    – Dave
    Mar 10, 2023 at 14:56
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    \$\begingroup\$ @Dave That might be an important RW consideration; RAW firing on a target you can't see but which you know the location of imposes disadvantage. Your siege engines (I'm assuming siege engines, not attacking spellcasters) will likely be at disadvantage for firing at long range anyway, and these disadvantages don't stack. \$\endgroup\$
    – Kirt
    Mar 10, 2023 at 16:06
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Diversifying isn't enough, the enemy needs coordination too. And even that can be countered by more bards.

As Thomas notes, the wall can only be immune to one form of damage at a time. But that's not a major limitation on this strategy; the Combining Game Effects rules allow you to constantly replace the immunity, as long as your have a sufficient number of Bards with Lyres to provide the reactions:

The most potent effect - such as the highest bonus - is the one that applies, or the most recent effect applies if the effects are equally potent.

So even if the enemy diversifies their damage, say, alternating between bludgeoning from catapults and piercing from ballistas:

  1. They need to perform this alternation with perfect synchronization during a six second round, and
  2. It just means you need more reactions (read: more lyre-toting bards) to continually switch the immunity between the various damage types

Frankly, at a certain point it gets kind of absurd to imagine the level of coordination needed here. Trebuchets and mangonels take five actions from beginning loading to firing; ballistas and cannons take three. You're not firing every round, and frankly, while D&D pretends this stuff runs like clockwork, with non-heroes loading and firing, I'd expect them to get out of sync to the tune of a few seconds (enough to ruin carefully timed attack order) quickly. To deal damage to such a structure, you'd need at least as many siege weapons (and/or spellcasters with spells that are equivalent to a siege weapon attack) as the defender has bards and you'd need to make all your attacks, in a single round, perfectly alternating so that two attacks dealing the same damage type don't occur back-to-back.

If you're reliant on magic, your casters will likely be out of siege weapon caliber spells after just a handful of rounds ("five minute workday" is underselling how quickly those higher level slots would run out). If you're reliant on a mix of real-world siege weapons, you can keep it up all day, but you can only pull off the coordinated volley of mixed damage every few minutes, and those bards can be Mending the damage while they wait; normal Mending has a one-minute casting time, so it's not a quick response, but the Lyre grants it as an action, so the bards can be both reactively defending and actively fixing small breaks in the walls live.

In short: Even if the game rules technically allow for it (carefully readying actions to fire dozens or even hundreds of siege weapons or equivalents in a single round to overwhelm the defending bards), practically speaking, the structure is immune to attack if you can keep a dozen or so bards available to defend it.

You need to prevent the reactions entirely

Alternating damage types in sufficient volume to overwhelm the defenders' reactions is a sucker's game. Lyres of Building are almost certainly more common than the high-level casters needed to coordinate an attack of the necessary magnitude in any vaguely realistic way; and if someone is all-in on this defensive tactic and can afford to build them, all they need is to train up a bunch of low-level bards to wield them, which is a lot easier than the enemy hiring/training enough high-level siege casters to overwhelm them.

So don't play it as a numbers game. Find ways to prevent the reactions, e.g.:

  1. Follow Kirt's advice. Don't use your casters to attack the walls. Bring regular old siege weapons, get them aimed on target (it's not like walls can dodge), then have your casters blanket the walls/defenders/ammunition with Darkness, Fog Cloud, etc., so no matter how many bards the enemy has, it doesn't matter, they can't use their reaction to defend. Or no casters, just bring along an Eversmoking Bottle (just uncommon rarity), and you can repeat this all day long (allowing 10 minutes between waves of fire for smoke to dissipate for aiming again), with no limited magical resource expenditure at all. Coordinate enough to fire in waves, focused on one section of the wall, both to minimize the number of obscuring spells needed, and to ensure enough damage has been dealt that Mending-as-an-action is useless (if the damage is greater than a foot in any dimension, Mending doesn't work). That'll leave them with their once-a-day spells for larger repairs, and if you keep up the siege weapon fire all day, with only a couple of lower-level support spells required per barrage, you'll overwhelm that pretty quickly.

    This doesn't even require much flexibility from your DM to allow it; the rules for siege weapons like trebuchets and ballistas already require the process of using the weapon to take multiple actions, split into loading, aiming and firing. You only need to see the target when aiming. You only need the target obscured when firing. For emplaced siege weapons, attacking large, immobile targets, it's perfectly reasonable to use the visibility conditions at time of aiming, not at the time of firing, when the two actions are only separated in time by 6-12 seconds.

  2. Use spells that destroy the castle without damaging it. Got Transmute Rock to Mud? That mud is unharmed, but much less useful, defensively. Got Move Earth? Take a few minutes to undermine the walls until they collapse under their own weight (arguably "damage", but not exactly "taking damage"). Animate Object a small (less than 15'x15') tower and have it just walk away (or start bashing the bards). RAW, Disintegrate doesn't do damage to objects, it just turns a 10' cube of it to fine grey dust directly, no damage involved, so use that. etc., etc. You get the idea.

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  • \$\begingroup\$ I'd like to better understand how the rules support "kirt's advice" In point 1 -- can you point out the rules that are relevant for allowing a siege weapon to "target" a section of wall and then no longer have their attacks be affected by the obscuration spell effects having done so? \$\endgroup\$
    – Dave
    Mar 10, 2023 at 15:02
  • \$\begingroup\$ Or better yet, ignore the fortifications alltogether and kill the bards first. The walls aren't going anywhere \$\endgroup\$
    – Negdo
    Mar 10, 2023 at 15:13
  • \$\begingroup\$ @Negdo: Sure, but those walls tend to get in the way. :-) \$\endgroup\$ Mar 10, 2023 at 15:40
  • \$\begingroup\$ @Dave: I thought this was going to be purely a DM judgment call based on "reality", but actually, there's at least some support for the idea in the DMG siege weaponry rules. Both trebuchets and ballistas take multiple actions to load, aim and fire. The "aim" action is a completely separate action from the "fire" action. If the extremely steady siege weapon is aimed at a large, visible, and immobile target, then the obscuring spell is cast, then six seconds later the weapon is fired, it doesn't take much DM leeway to say the attack is tied to the conditions at the time of aiming. \$\endgroup\$ Mar 10, 2023 at 15:51
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    \$\begingroup\$ @Dave: Yep, that's reasonable. But you can still set up a dozen siege weapons aimed at one spot, obscure it, fire, drop the obscurement spell, repeat until you run out of slots. If you're a little more patient, and have access to an Eversmoking Bottle (uncommon rarity; if you are going up against a target that can equip a platoon of bards with a rare magic item each, I'd hope you can afford a single uncommon item), you can aim, uncork, wait a few rounds (depending on how far the smoke must spread to conceal the wall), attack, close the bottle, repeat 10 minutes later when the smoke dissipates. \$\endgroup\$ Mar 10, 2023 at 16:08
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Invulnerable to damage doesn't mean impenetrable

The idea of attacking walls is simply to make a way for the attackers to enter the city, not because they have something against walls (necessarily).

Assuming the opposing army is composed of a host of powerful spell casters too, they can cast something like stone shape which will let them alter the stone to form a door. This doesn't damage the structure and has no damage type so isn't preventable under the effect of your Lyre of Building.

Don't assume an enemy will use real-world medieval siege tactics in a magical fantasy world (especially one where magic is so common that an army of bards with rare magical items protect a city without bankrupting it).

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Enemy Bards

Professor Farnsworth said it best. Nothing can stop a giant monster.

Futurama GIF: The professor says "Except an even equally big monster."

Likewise, nothing can stop your army of bards. . . .

Except enemy bards bards with their own Lyre of Building. To quote your question:

you can play the lyre as an action to cast Fabricate, Move Earth, Passwall. . .

A single enemy bard can stroll up to the wall and cast Passwall or Move Earth to make a tunnel and march their army through.

These spells do no HP damage to the wall. So you Lyre cannot prevent the spell.

Checkmate.

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  • \$\begingroup\$ This is where the wizards with counterspell readied come in... \$\endgroup\$
    – User 23415
    May 2, 2023 at 20:03

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