Huh, this kinda reminds me of an old Scratch game I made a while back (where you build a fortress in between rounds, and defend it from enemies during rounds, but instead my one had the crystal be controlled during rounds to shoot down enemies. I actually almost chose to make a slightly different, improved version of that game for this jam, but later favored another idea after not only the scope getting too big for it, but also my teammate agreeing on the other idea too).
Good execution, just wished there were more blocks (buyable by clicking on their icons instead of pressing specific keys), more opportunities to earn coins, and slightly faster rounds (since you do basically nothing during them, and you can offset quicker round timers with more aggressive and faster spawning enemies after all). It also wasn't mentioned anywhere if I recall correctly that the game ends if the crystal touches the ground either.