Does Dispel Magic work against the Mighty Fortress spell?
My GM and I couldn't decide what to do.
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If you look at the spell description of Mighty Fortress (XGtE, p. 161), you can see its Duration as a spell:
If you then have a look at page 203 of the Player's Handbook, you'll see the following under the descriptions of Spell Durations:
Many spells are instantaneous. The spell harms, heals, creates, or alters a creature or an object in a way that can't be dispelled, because its magic exists only for an instant.
So, even though the structure created by Mighty Fortress crumbles after 7 days, the spell that creates it is Instantaneous in duration, and is thus immune from Dispel Magic.