So I built a rather large castle on Spectacle Island and now I want to demolish it and start over. Anyone have any ideas how to do this in one fell swoop, rather than having to spend 45 minutes taking it apart brick by brick?

Highlight one of the bricks and instead of selecting it, hold down the button. It should auto-highlight all or most of what you build, and you should be able to scrap/store all of it from there.