You guys been doing a good job putting this together. The art and music are on point and the tower defense fundamentals work pretty well. As for your questions:
1. I think building would be a bit more intuitive if you switched the control scheme for placing towers to be a two click system where you click to select the tower and then click again to place it. The drag and drop can be a bit unreliable when you're scrambling to build towers.
2. I wasn't aware of the upgrade system initially so I think making that more apparent and clear on how to upgrade is an improvement to make. I also think tooltips on enemies that will appear next round telling the player their type and their effects would be nice. I didn't exactly know what enemies were flying types at first and having that info to know what towers to prep for the next round would be helpful. Then also knowing that the stingray is actually a cruise missile that destroys my hopes and dreams is also some info I would've loved to know too.
3. The game comes out swinging and in these initial play throughs I found it a bit challenging while I was learning the tower mechanics. It becomes easier after learning what to do but I think toning down the difficulty in the first few rounds just by a little would help. I found myself getting jumped in round 9 only to get destroyed by the Leviathan the round after. And I think a little more time to get established would make this game feel good. Then after round 10 I'd say it feels right to get funky with the difficulty. I do like the inability to build towers on top of destroyed sites. It really adds a layer of challenge and strategy. If you'd want to make games longer I think being able to put out fires for a cost to restore a tile would be a good addition, maybe make the cost to do that very expensive to prevent people from overly relying on it. Just a suggestion though.
4. I liked the tower selection. I think you have a good set of towers and the tooltips that tell you what they do and the enemy types they combat is good. I don't think you need to change any of that as I think everything you need to hold out is given to the player.
One last thing for you guys, in one of my games I lost to the Leviathan in round 10 and had a game crash at the very end. Game still ended but was unresponsive at the name input screen. I'd look at that. I'll attach the webpage error but I would think there's some Unity error that caused this to be thrown.
Outside of that this game is coming along real well, good job so far guys!
-Micco Estrada