How do spells that destroy objects interact with structures?

Question

Per the answer to "Are objects structures and/or vice versa?", the rules on objects state (DMG, p. 246):

For the purpose of these rules, an object is a discrete, inanimate item like a window, door, sword, book, table, chair, or stone, not a building or a vehicle that is composed of many other objects.

implying structures are considered a collection of objects.

Now, certain spells like shatter, delayed blast fireball, and meteor swarm specifically say that they also damage objects and not just creatures. From the description of meteor swarm:

A creature takes 20d6 fire damage and 20d6 bludgeoning damage on a failed save... The spell damages objects in the area and ignites flammable objects that aren't being worn or carried.

As a frame of reference, each wall of the instant fortress magic item has 100 HP, and a 6-inch thick panel of the spell Wall of Stone has 180 HP, all of which can be destroyed with a couple casts of aforementioned spells. Does this mean any wizard with these spells would be able to level any city or even a castle within a span of two or three days? That seems awfully fast.

Are there any rules I missed regarding destroying structures from the rules?

"not a building or a vehicle that is composed of many other objects." Did you answer your own question? — guildsbounty, Oct 30 '20 at 22:02
fwiw, the 1-brick-thick walls you see on a lot of movie and video game castles is wildly inaccurate to history. A typical castle wall would be somewhere upwards of 10 feet thick (yes, including the walls of towers where archer's slits would be), and could go up to 24 feet thick. If you just use Wall of Stone for reference...you're looking at 600 to 1,440 hp for a single segment of castle wall. Max damage on Meteor Swarm wouldn't drop a single chunk of wall. — guildsbounty, Oct 31 '20 at 00:37
Castles are very hard to totally destroy even with gunpowder https://en.m.wikipedia.org/wiki/Slighting — Dale M, Oct 31 '20 at 03:53
Related: How should damage thresholds be assigned, and Hp and damage thresholds for defensive structures — Kirt, Nov 09 '21 at 04:49

score 7 · Accepted Answer · answered Oct 31 '20 at 01:39

As you mention, the rules describe what an object is and what it is not e.g. vehicles and buildings.

Page 247 of the DMG refers to the concept of a damage threshold:

Big objects such as a castle walls often have extra resilience represented by a damage threshold...

This implies, to me, that you can never target a building such as a castle with a spell, only one or more objects that make up the castle. So targeting the portcullis or the wall is acceptable and would eventually lead to that object being destroyed, with that possibly being a step towards the castle coming crashing down.

Destroy enough objects that make up a structure and it will fall but you can't destroy the structure itself outright.

Your Wizard would be kept very busy destroying every supporting object before they could claim they've 'destroyed' the castle.

How do spells that destroy objects interact with structures?

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