really fun game! my only couple of criticisms would be -
with regards to the difficulty scaling, it really feels like the difference between 'easy' and 'normal' is in the later half of the game? for me at least i wish there was a mode that could ramp up to the hardness - it's just when i'm in the early part of the game i really need to get my bearings/get on solid ground. also - it would be cool if there was an upgrade to show how close you were getting to the core/how many levels down it is, or something else that at least reminds you what the objective is. maybe it's a little silly, but i got caught up so much in the process of mining and defense and upgrades that i totally forgot the goal was to get to the core, and got a little confused/frustrated once i'd upgraded everything and wasn't sure what to do next.
really cool design, though - it's very cute and i love the colors, and the gameplay is one that imo immediately pulls you in!