What constitutes a freestanding structure? The definition of freestanding is "Not supported by another structure. Not relying on or linked to anything else; independent." By this definition you could not target a mosiac on a building wall, or even the wall itself, but could you target the building instead and damage everything including walls and said mosaic? Freestanding seems to be partly a matter of perspective.
Another example: You can't target the door to a watchtower, but you can bring the whole watchtower crashing down.
Is this the correct interpretation of this spell?
There are no limitations on size or complexity in the spell definition. It just seems weird that if you target a castle portcullis the damage gets absorbed by the surrounding stone, but if you target the castle itself you can bring down the whole structure.
There are no limitations on size or complexity in the spell definition. It just seems weird that if you target a castle portcullis the damage gets absorbed by the surrounding stone, but if you target the castle itself you can bring down the whole structure.
A 20th level bard can do 40d10 damage (avg. 220) in total over 20 rounds; I doubt that's enough to destroy an entire castle, considering a single 10'x10' section of masonry wall has 90 hp.
