Thank you so much for playing all the way through 0.3.0 and for taking the time to write this — it's exactly the kind of feedback that shapes the game.
Your leftover stockpile (151 copper vs. 4,552 tin and 4,199 iron!) says it better than any chart: tin and iron run out of uses at the end. The plan is a new ore, with its own talents and new buildings for the fortress, so your tin and iron have something to go into.
You were right about the bomb, too. It now goes off in a single hit, the lightning sets it off instead of stopping on it, and the blast is wider, so it works with your lightning instead of against it.
About the levels: the game now explains them. Break enough rocks on a floor and you drop to the next one, where rocks are tougher but give more ore. A counter under the floor gauge shows how many are left, and each new floor shows its bonus. If by "levels" you meant something else, please tell me!
Achievements with a small reward are next on the list, and more bosses are on the way. Glad you liked the lightning, the fortress, the merchant and the foundry.
Thanks again!