I’m personally not really big on tower defense. I think your game was enjoyable. You seem to be at the start of nice.
I liked the hexagonal grid. The placement of towers was clear and the radial display of their threat zone made it very easy to decide what to do.
The UI around upgrading towers could use some improvement. There’s no real hint that upgrading towers is how you use the crystals. Instead of having a popup that gets bigger to expand as you level up, I think it would be better if your upgrade UI showed what was possible at various levels. That way the player can think about their investments and decide if they want to really upgrade a tower or place more of them.
The waves of enemies weren’t really challenging for a long time. They started ramping up in difficulty right around when I lost interest in playing further. You might want to consider adding an earlier ramp to the difficulty.
I found that the soundtrack got tiresome after a while. It would be nice to have a way to turn that off, or if it could have more variety.