The party ran across an inconvenient stone castle, complete with large moat. But we had a Monk with Way of the Four Elements:

Elemental Attunement
You can use your action to briefly control elemental forces nearby, causing one of the following effects of your choice:

  • Create a harmless, instantaneous sensory effect related to air, earth, fire, or water, such as a shower of sparks, a puff of wind, a spray of light mist, or a gentle rumbling of stone.
  • Instantaneously light or snuff out a candle, a torch, or a small campfire.
  • Chill or warm up to 1 pound of nonliving material for up to 1 hour.
  • Cause earth, fire, water, or mist that can fit within a 1-foot cube to shape itself into a crude form you designate for 1 minute.

PHB p. 81

My Monk used this to dig little tunnels from the earth beneath the moat that spread under the walls. Each action, the monk forms a cubic foot of earth to have several tunnels (probably ~1 inch in diameter). The tunnels immediately fill with water (from the moat), so they last beyond the one minute duration.

After a couple of minutes (depending on the exact dimensions of the moat), the castle collapses (or the front wall shears off, or the entire castle tips forward, depending on exactly how we shape the tunnels). The defenders have no way of knowing what's going on, and no way to prevent it.

Does this really work?

  • \$\begingroup\$ Welcome to RPG.SE! This is a solid first question. Please take a look through the tour when you get the chance, but it seems like you've got the jist of this. \$\endgroup\$
    – UrhoKarila
    Commented Oct 31, 2016 at 14:06
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    \$\begingroup\$ RE: "The defenders have no way of knowing what's going on." Y'know, except for that dude standing near the castle, casting a spell over and over! (That is, were the castle's defenders also somehow oblivious to the PCs' presence?) \$\endgroup\$ Commented Oct 31, 2016 at 14:08
  • \$\begingroup\$ @HeyICanChan: But there is no visible connection between the caster and the spell effect, and the spell has no fixed range limit. The monk can use all terrain available to fully hide from the castle (or just ask a Wizard to cast Invisibility). \$\endgroup\$
    – SLaks
    Commented Oct 31, 2016 at 14:28
  • \$\begingroup\$ Why would you ask a question and then argue over both offered answers? \$\endgroup\$
    – Zeiss Ikon
    Commented Oct 31, 2016 at 15:58
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    \$\begingroup\$ "strong enough to prevent this from working" ... Other than the gold rule? You are looking too far. \$\endgroup\$ Commented Oct 31, 2016 at 17:21

7 Answers 7


Your monk is relying on physics while simultaneously ignoring physics.

Your monk is attempting to use physics in a game with magic. If you're going to permit it, remember to apply all of the rules.

The amount of pressure that much stone has on the dirt means any channel the monk creates will immediately be filled by the compression of more dirt. Any water attempting to fill the channel will be immediately squeezed out.

As others have pointed out, castles aren't built on dirt specifically because flood waters and heavy rains cause serious problems. This means the foundation for the walls will either be resting on bedrock, or have a wide footprint allowing for a distribution of weight at a level that resides below the frost line.

Either way, the ability is not going to be effective in laying siege to a castle wall.

Lastly, the monk needs to be able to see the specific spot he's choosing to effect, and if he's remaining in cover several hundred feet away in the forest, this should definitely be an impossible perception check. Unless he has a laser range finder, he's going to need to be a LOT closer than that to pinpoint the spell. No spell has an "about there-ish" target mechanism. If he's extremely argumentative, I'd ask him to specify in feet exactly where he was going to use the spell. He better be bang on the first time, because people are going to start questioning the sudden appearance of dirt furrows in the middle of the road if he over or under shoots.

So you can counter this several ways:

1) Physics

2) Engineering

3) Line of sight

  • 9
    \$\begingroup\$ All spells that require a target require line of sight unless otherwise specified. \$\endgroup\$ Commented Oct 31, 2016 at 16:45
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    \$\begingroup\$ To add a source for Lino Frank Ciaralli, the rule he's mentioning is on page 204 of the PHB. \$\endgroup\$
    – UrhoKarila
    Commented Oct 31, 2016 at 16:50
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    \$\begingroup\$ Lino, it isn't a spell. It is a class feature, not a cantrip. \$\endgroup\$ Commented Oct 31, 2016 at 17:26
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    \$\begingroup\$ @J.A.Streich - Fair enough. Even granting that, he still needs to be able to guesstimate the range. It's not like it's a laser range finder. \$\endgroup\$ Commented Nov 1, 2016 at 1:06
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    \$\begingroup\$ To add to this, I don't think hiding in a forest some hundred meters away would classify as "nearby". For me, "nearby" would be like... 3-5 meters (10-15 ft, 1-2 fields) at most. Thus, standing in the middle of the moat, and clearly visible for anyone on the castle wall willing to shoot down some arrows. \$\endgroup\$
    – Damon
    Commented Nov 1, 2016 at 7:54

Elemental Attunement is a very minor effect equivalent to a cantrip, made available to a class not normally magically inclined. Any consideration of game balance must conclude that such magic isn't capable of anything like undermining a castle wall (at least not in any reasonable time frame -- over a lifetime, perhaps, but not in a week or even a month).

Proper undermining, done by sappers, involves digging timber supported tunnels under the full width of the wall over some distance, then burning out the timbering in a matter of, at most, hours. Even so, the mining part takes weeks or months for large crews of trained workers. This was only ever a siege technique.



Form Not Long Enough

The tunnels only last a minute, and you can only do it 10 times in that minute. At most you can move 10 cubic feet of dirt per minute, and it magically goes back to normal at the end of that minute. So, at any given second, you 10 cubic feet of tunnel. Not enough to topple a wall.

Not Enough Water

It can create a cubic foot of water per round (6 seconds). That's actually a lot of water in volume, but I am not convinced that 10 cubic feet of water a minute... over a 5 foot area wouldn't just disperse.

Doing some quick math: 10 cubic feet of water ~= 237 gallons (/minute) A fire house puts out 95 gallons/minute That means you can create less than three fire hoses worth of water, which isn't uncommon in fighting common house fires, and you're doing in cubes of water similar to buckets.

It is unconvincing you could topple a castle with that flow rate, fast enough not to become exhausted from lack of sleep, or be discovered by people noticing the muddy ground.

Moral of the story: Don't bring firefighters to an assult against a castle.

Clearly Outside RAI

The clear intent is that this effect has utility (outside damage and combat) and role playing effects. It doesn't list damage dice. RAW, the wall has hit points and AC like all other objects and creatures. The AC is ridiculously low, but the walls hit points are ridiculously high. This abstraction means you have to deal damage to it. As written this class feature lists no damage dice, and thus isn't intended to do damage. Otherwise you could impale people by making spikes of earth under enemies feet, but clearly if you could they would list what the damage for such a thing should be.

  • \$\begingroup\$ I'm not creating water; I'm draining water from the moat. The moat has more than enough water. \$\endgroup\$
    – SLaks
    Commented Nov 1, 2016 at 0:17
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    \$\begingroup\$ Obviously it doesn't... It is already sitting on the earth. If it were enough to saterate the ground and sink the castle, it would have already. But even so the volume of water the tunnels hold is still equal to the volume you'd be able to create, so the water argument is still the same even without creation. \$\endgroup\$ Commented Nov 1, 2016 at 3:27


  1. The dirt is not destroyed, it has to go somewhere, and will revert in one minute, filling your tunnels back in.
  2. 1" tunnels under minimal pressure won't actually carry all that much water. At least some portion of what they do carry will end up working towards collapsing your dirt tunnels.
  3. Castle walls tend to be thick, and very heavy. They do not rest on bare earth, but instead on natural rock, or are sunk fairly deep into the ground so that they don't fall over the first time there's a heavy rain. Making the surrounding soil slightly damp will not be sufficient to destroy the wall.

At best, you've just taken the first steps to create an irrigation system for the castle. If you want to actually collapse the castle walls, you're going to need to work a bit harder to undermine them. Repeated castings of Stone Shape would probably do the trick. Move Earth might also work.

In general, castle walls are meant to stop armies, not small groups of adventurers. There are any number of ways (both magical and mundane) to bypass walls, and once you've cleared out everyone inside, you've got a free castle!

  • \$\begingroup\$ 1) No; the water will fill the hole immediately, so the dirt won't be able to revert. The hole will lose its cohesion, so the water will saturate the area, turning it into mud. \$\endgroup\$
    – SLaks
    Commented Oct 31, 2016 at 15:37
  • \$\begingroup\$ 2) This will probably also draw more water from the moat above as the water spreads, further weakening the dirt. \$\endgroup\$
    – SLaks
    Commented Oct 31, 2016 at 15:38
  • \$\begingroup\$ 3) If the castle is on a large slab of rock, this won't work it all. If it's merely sunk deep into the earth, I can extend the tunnels arbitrarily deep (it may take time, but it's completely safe time). Mountain dwarves would be helpful here. \$\endgroup\$
    – SLaks
    Commented Oct 31, 2016 at 15:41
  • \$\begingroup\$ @SLaks, although I have not seen a word on that, the ability says «nearby forces», so that would preclude «arbitrarily deep» as one aspect of the spell. \$\endgroup\$ Commented Oct 31, 2016 at 15:44
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    \$\begingroup\$ Why are you assuming that the water will keep it from reverting? Where it that rule coming from? It's magic. Were I the DM, I'd rule that after a minute, the water would be squirted out after the earth reverted to it's original form. \$\endgroup\$ Commented Oct 31, 2016 at 17:47

The cantrip can shape the earth, but doesn't remove material, so there would still be the same amount of dirt supporting the castle walls. Secondly, one cubic foot at a time is slow going; the castle wouldn't suddenly fall down, it would gradually sink if this happened.

Thirdly, I'm not sure why he thinks creating tunnels in the dirt will somehow fill them with water from the moat above, nor why this would cause a loss of structural integrity in the foundation of the castle adjoining the moat. Compacted dirt with water tunnels through it can support the load just as well as the original dirt. Again, it's not like the amount of dirt has changed, it's just been reshaped.

  • 2
    \$\begingroup\$ 1) When the minute expires, the dirt will lose its spell-induced additional cohesion, and the water will saturate the area. This should weaken the dirt enough to break the castle's support. \$\endgroup\$
    – SLaks
    Commented Oct 31, 2016 at 14:57
  • \$\begingroup\$ 2) To make the effect less gradual, start from multiple points along the moat, and spread horizontally in both directions, so that there is still a bit of support along the wall until the last minute. \$\endgroup\$
    – SLaks
    Commented Oct 31, 2016 at 14:58
  • \$\begingroup\$ 3) The tunnels would start directly from the moat. Each tunnel opens at the surface of the dirt floor beneath the moat, so the water will naturally sink into them. The point is to have the water weaken the dirt. \$\endgroup\$
    – SLaks
    Commented Oct 31, 2016 at 15:00
  • \$\begingroup\$ It is not a cantrip, it is a class feature. \$\endgroup\$ Commented Oct 31, 2016 at 17:07

If you rule that the castle is resting on soft earth, and that the water will hold the tunnels open after the spell's duration expires, then yes, this will eventually work. But there are a couple limitations:

  1. This is a pretty slow process. Castle walls are big and thick, and adjusting one cubic foot at a time until you eventually tunnel down underneath the wall will take a while.

  2. Your average castle occupants will be very suspicious of anyone loitering. Even if they're not able to directly connect him with anything untoward happening, he'll likely be challenged and run off if not able to provide a good excuse.

  3. Even if he does breach the wall this way, castles are pretty well-defended. They probably have a number of archers and soldiers, and are likely built on a hilltop or other defensible position. A few people will have a tough time taking a castle even if they do manage to breach the wall.

None of which is to say it's not worth the attempt. Your monk would essentially be a sapper, which is one of the standard techniques for undermining (so to speak) a castle's defenses. But there are also defenses against this technique, some of which will still work on your monk, so it's far from foolproof.


I would not allow this to collapse a castle.

This uses an action. At the most, working fast, you can mold 10ft of "tunnel" (which would have to be smaller than a foot as you will need to compact the cubic ft of earth outward to allow it) before the first effect returns to it's original shape.

For the sake of argument, I will allow that your supposition that the earth at this point will become saturated.

However, to continue your endeavors, you would either have to move on to another tunnel (taking another minute to open up a 10ft tube) or reopen your original delving.

If you move on to another tunnel, you will conceivably have a very small section 10ft long and inches wide saturated, and be working on another. This will take a lot of time (a minute per ten feet). Opening any earth further in (beyond 10ft) would not result in any further saturation, as the water is no longer flowing in once the original effect closes.

If you stay on the same tunnel, you might expand the saturation a bit over time. This will not be sufficient to weaken a significant portion of the foundation.

Finally, in either case, there would be the effect, over time, of the ground drying out. If this is earth, the water will eventually drain away through it. The same way it drains away when it rains.

A castle is big. It will take time to create enough 10ft tunnels to fully saturate the ground around the castle, and after a minute, that specific area starts drying out.

So - No, it wouldn't work. I suspect after constant stealth rolls and/or someone wondering where the water in the moat is going, someone is going to notice there's an issue and send out patrols. Invisibility is no guarantee you won't be spotted. you might be leaving muddy footprints...


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