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The relevant part of Forcecage is:

[...] creating a solid barrier that prevents any matter from passing through it [...]
[...] any creature that is completely inside the cage's area is trapped.
A creature inside the cage can't leave it by nonmagical means.

The spell (i.e. it's magic) Thunderous Smite says:

Additionally, if the target is a creature, it must succeed on a Strength saving throw or be pushed 10 feet away from you and knocked prone.

Emphasis Mine.

The scenario is, smiting somebody using Thunderous Smite, such they they would be pushed through the bounds of a force cage.

Now, I'd never assume that shoving a creature through non-magical means would work, that seems clear. But I'm struck by the visual of smiting somebody with a magically enhanced strike so hard they pop out the side of a force cage. The pushing is obviously magical, and non-teleportation effect can leave the cage without a charisma check. The last sentence implies those trapped can use magical means to leave, potentially bypassing the previously listed limitations. I have a glimmer of hope that this magical pushing somehow gets around the forcecage, in either cage or prison mode.

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The rules assume you already know how walls work

I've looked everywhere I can think of, and as far as I can tell there's nothing in the rules explaining that you can't walk through walls.1 Despite this, I've never met or heard of a DM that allows their players to walk through walls. Walls are commonly understood by everyone to be barriers to movement, in addition to providing total cover. This includes both voluntary movement, such as walking and flying; as well as involuntary movement, such as being pushed, pulled, or carried; but generally not teleportation and similar effects that allow one to arrive at their destination without movement.

(It might seem obvious or even a bit patronizing to go through all of this, but it's important to establish it as a baseline that we all agree on, since as far as I can tell "walls block movement" isn't actually an explicit rule. Also, JC has stated in a tweet that walls "work the way we expect them to, except for when the rules say otherwise.")

Magical pushing still causes physical movement

So if we take it as a given that walls are impassible, the question becomes whether there is anything special about a push effect caused by a spell or magical effect, compared to something like a mundane shove attack. In general, I would say there is no difference. Just because the thing that launched the target backward is magic doesn't make the movement itself magical. Even if you were to pick up the target with a Telekinesis spell and try to carry them through the wall, it wouldn't work, whether the wall belongs to a castle or a Forcecage spell. Regardless of the force doing the pushing, the target is being physically moved, so any barrier to physical movement will block them.

You need the right "magical means" to escape a Forcecage

The Forcecage spell says:

A creature inside the cage can't leave it by nonmagical means.

This establishes that you can't escape the cage by hacking your way through the wall with a axe, picking a lock, using a crowbar to separate the bars, etc. However, it doesn't mean that any magic spell or effect will get you out of the cage. Even when you're using magic, the wall of the cage is still a wall. You're going to need a spell that can get you through a wall. Teleportation is one way, subject to the limitations described in the spell. Disintegrate can also punch a hole in the cage for you. However, Passwall isn't going to help you, because the cage isn't "wooden, plaster, or stone", and Etherealness won't get you out either, since the cage extends into the Border Ethereal. There may be other spells that can get you out, but the point is, a spell will only allow you to escape the cage if it helps you pass through a wall made of magical force.


1The closest I can find to an explicit statement that walls are impassible is this rule from the grid variant rules:

Corners. Diagonal movement can’t cross the corner of a wall, large tree, or other terrain feature that fills its space.

There is also this excerpt from the DMG about movement in the border ethereal plane:

[...] solid objects on the overlapped plane don’t hamper the movement of a creature in the Border Ethereal.

Both of these assume that walls are impassible, but don't say so on their own.

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    \$\begingroup\$ Yup, I'm definitely not trying to assert that I can walk through normal walls as the rules don't prevent it. But a magic wall seems like it could be different. \$\endgroup\$ Dec 29, 2021 at 22:19
  • \$\begingroup\$ Technically, wouldn't etherealness allow someone to escape the forcecage if the analogous spot in the Border Ethereal to their location on the Material Plane was a portal to the Deep Ethereal? \$\endgroup\$
    – nick012000
    Dec 30, 2021 at 0:20
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    \$\begingroup\$ That sounds like a separate question about the interaction of Forcecage with existing planar portals. \$\endgroup\$ Dec 30, 2021 at 2:18
  • \$\begingroup\$ This makes sense if you choose the box prison option. That is essentially a wall. But what about the cage prison option? It isn't described as solid, and does not say matter or magic can't get through. Also, what does Thunderous Smite say about barriers, and does it mention them needing to be solid? \$\endgroup\$
    – trlkly
    Dec 31, 2021 at 6:26
  • \$\begingroup\$ @trlkly The wall and cage are equivalent for the purposes of this question. Both are barriers that can't be escaped by nonmagical means (even if you're small enough to squeeze through the bars!) So physical movement can't be used to escape either one. Few, if any, forced movement effects say anything at all about what happens when the movement is blocked. It's left up to the DM. In the case of a weak wooden wall, you might allow forced movement to break through the wall. But that wouldn't make sense for a wall of magical force. \$\endgroup\$ Dec 31, 2021 at 6:35
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Tree stride and transport via plants are “magical transportation,” to quote tree stride, even if they aren’t “teleportation.” (Transport via plants doesn’t use that phrase but it has nearly the same effect so I think it’s fair to state it applies.) That is, they are magical instantaneous changes in your position without crossing through intervening media—including the wall of forcecage. They may not interact with effects that explicitly interact with “teleportation” but in the colloquial sense that’s what they are.

Anyway, thunderous smite “pushes,” it doesn’t “instantaneously relocate.” The pushed creature moves through the intervening media—as much as it can. It can’t move through the wall of a forcecage, so it just stops.

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    \$\begingroup\$ I think this is the necessary interpretation, if only to avoid questions like "can I walk through force cage if I'm wearing magic boots?" and "can I walk through force cage if my speed is magically enhanced, such as with haste?". \$\endgroup\$ Dec 29, 2021 at 15:49
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    \$\begingroup\$ @ThomasMarkov yup, this makes sense to me! \$\endgroup\$ Dec 29, 2021 at 15:53

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