The implications of gunpowder re: steampunk

I was watching an episode of the Nerdwriter just now in which (the narrator dude whose name I can't quickly find on his page) argues that the existence of dragons is the reason that the world of Game of Thrones never industrialized.

The main point is that cannons are the thing that break the castle system of self-defense, and since you can buy cannons, once they're available they stimulate rapid economic growth.

This is something I've thought about a lot, because I play D&D sometimes and am a huge nerd and tend to write characters who are at least a little bit irritated that everything is done so inefficiently considering there's literally magic to work with. One of the questions that occurred to me while I was developing one of those lines of thought is whether there were any pre-gunpowder chemical explosives, which, it turns out, no. There weren't.

So I started to try to work out how you could break down castle walls, and I thought, "Compressed air." So I googled air compressors, which it turns out require motors that run on either combustion or electricity.

So that's out, unless I can come up with a different kind of engine, which was when I remembered steam engines, then realized that a steam engine is a compressed air generator, then realized that goddammit I just accidentally re-invented steampunk.