Chapter 2 and The Fuse - that is further than most people get, thank you for actually playing it through.
The Fuse is the one I fought with longest. It only works if the fire takes exactly as long to crawl the line as the player expects, and "about right" is not good enough there - a quarter second either way and the whole level reads as broken instead of tense.
You are right that the solver is the reason the levels hold up: every one of the 15 hand-made levels and the 200 generated towers is proven winnable by its own recorded solution before it ships, so no level can quietly become impossible after a balance change. It catches me more often than I would like.
Good luck in the jam to you too.