1. I felt the process of buying and placing the towers was intuitive. I didn't really know the range of any of them. When the towers are all on the board it is hard to keep track of which one is doing what. The act of upgrading is kind of intuitive but I didn't have a need to hover over any of the towers so if I had not read your description I wouldn't know that I could upgrade them.
2. I think knowing the range each tower has when placing them (maybe in a circle like you did for one of the towers) and when they are actively attacking enemies would be helpful. Also, it would be helpful to know how much damage each tower has taken and how much each tower gives. I know the towers flash red faster the more damage they have taken but that is only visible when an enemy is currently attacking them. It would be nice to know how many more rounds until the Leviathan comes. I know it is every ten rounds but it would be nice to have a visual reminder on screen (maybe a countdown). I think this will help remind the player about how much time they have left and will also make the rounds leading up to the Leviathan feel more nerve-wracking.
3. I feel like the first ten rounds until the first appearance of the Leviathan were pretty easy and then the shift to the next appearance the rounds got really difficult. Having the rounds more difficult made it a lot more fun. I think the first ten rounds should get progressively harder (so round one would be easy but by round ten it would be harder) instead of waiting until after the Leviathan shows up the first time. Otherwise, I felt like there was a lot of downtime in the first ten rounds. The difficulty after that is good. It feels rewarding to try to play for as long as possible.
4. The central tower's health was noticeable since the center grew more red. I noticed the towers flashed red faster depending on how much damage they received but if there was another indicator I didn't notice it. I think when the towers were being attacked the red flashing was enough to know how much damage the tower had taken. However, if the tower wasn't being attacked it would have been nice to know the health for it.
I didn't notice any bugs. I love the art style and the noticeable difference between the different towers and how they play. Great job! I look forward to seeing how you continue to develop and improve the game!