The smelter needs more upgrades. I gave up before getting the last set of upgrades because I didn't like having to go through six loads of resources just to upgrade it once. Also, I had everything but that upgraded before even unlocking the final area.
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I liked it. I think it needed more depth, like more kinds of ores, but I suspect that's already planned. It would be nice to be able to upgrade digging distance and jumping height. More jump height would make it safer to mine without having to worry about not being able to get back. By the end it feels like you can't even dig up because the explosions keep bringing you deeper.