I had a great time with the demo, a combination of classic tower defense with Dome Keeper-esque planning feels very good.
Got some bugs I found, for efficiency I got all three of them in one screenshot:
1. You can drill past the surface and ores still generate in the air. You can't place drills or mines there, though.
2. You can place tiles disconnected from the main structure if they're connected to the mid-air ores. You can place turrets in these tiles.
3. A tier 3 Advanced Drill skips over the first tile without clearing it. [And before you ask, this also happens underground.]