Hey!! Kudos on getting this complex game up and running! I did 24 waves, so there is something grabby about your game for sure. I really like the idea of the elemental currencies and combining them in different ways. That's a cool unique idea!
Tower defense games really depend on balancing and design. BALANCE: Here the balance is a little too easy, so I don't really have to think about what I'm doing. I just put towers down and upgrade them a lot, and I can play for a long long time without ever losing any life. In a good tower defense game you should worry about dying in the first 5-10 waves. You should be forced to make a specific plan to deal with specific enemy type. Often there are suddenly a bunch of fast enemies that force you to slow them down. Something like that. DESIGN: There are a lot of things the player needs to understand in a tower defense game. What do all the towers and enemies do? What would strategic and non-strategic decisions look like? Currently I can't really tell in this game. I can't really tell what the different towers do, if they have different ranges, different effects, so I don't know why one would be better in one place or another. I don't know how the different enemies are strong or weak. In a game like this I need all that info so I can make a plan. Currently the clearest tower is the magma tower, because I can see exactly which tiles it affects and when. Its range and attack frequency are very clear, though I'm not exactly sure about its effect. Damage + Slow??
Another important issue here is that upgrading currently feels totally unrewarding. Mostly I don't see anything different and I can't even see damage numbers go up. I want to see those towers get bigger and badder! Instead of having lots of cheap upgrade levels, you should consider having just a few (3-10) , but upgrades that cost a lot more, so when I make an upgrade it feels big and meaningful. I should be saving up, and hoping to upgrade a tower just in time to save the day.
Have you played the Kingdom Rush tower defense games? I think those are some of the best ever made and worth checking out.
Nice work on this! I hope you keep working on it!