Is Daern’s Instant Fortress considered an extra-dimensional space?

Question

Some context: myself, and another dungeon master were discussing Daern's instant fortress and its interactions with a bag of holding.

• My stance was that the instant fortress is essentially utilizing a very powerful enlarge/reduce spell.
• And his stance was that the instant fortress is an extra-dimensional space, and thusly, would not interact positively with a bag of holding, or any other massive capacity storage device.

We were discussing this due to a player wanting to create a mystical item that combines the effects of the instant fortress and the bag of holding to allow for the fortress to have things inside of it even when reduced.

Is Daern’s Instant Fortress considered an extra-dimensional space?

Answer 1

No, it is not an extradimensional space

Instant fortress does not say it is or contains an extradimensional space. It says:

You can use an action to place this 1-inch metal cube on the ground and speak its command word. The cube rapidly grows into a fortress that remains until you use an action to speak the command word that dismisses it, which works only if the fortress is empty.

The cube rapidly grows into a fortress. That is, the cube already is in this dimension, it just grows. Compare this to other objects like bag of holding, handy haversack, or portable hole: they say that they are larger on the inside than from the outside, and they also reference extradimensional spaces, here for example from handy haversack:

This backpack has a central pouch and two side pouches, each of which is an extradimensional space. [...] Placing the haversack inside an extradimensional space created by a bag of holding, portable hole, or similar item instantly destroys both items and opens a gate to the Astral Plane.

If you homebrew a new item, that combines the features of one of those with instant fortress, for example by allowing items to remain within the fortress when you shrink it by putting those items into an extradimensional space, how that item works is up to you and the DM you work with. One way could be to use similar language (e.g. by putting the "instant fortress of holding" into another extradimensional space, or a bag of holding into it when it is shrunk, they both get destroyed and open a gate to the astral plane.)

Answer 2

The fortress cannot be put away if there are things inside it.

The item description of Daern’s Instant Fortress states:

The cube rapidly grows into a fortress that remains until you use an action to speak the command word that dismisses it, which works only if the fortress is empty.

You must clean out the fortress completely before you can put it away, so it cannot be used as storage.

As for the fortress being an extradimensional space, the description says nothing of the sort, so it is nothing of the sort.