The concept is good but the game is very hard at the moment, especially due to the lack of option you have. Making even one small mistake will snowball the monsters into your castle very quickly. But it has promise if you develop more content for it and improve the balancing.
Some quality of life things i'd like to see if you decide to keep working on it:
Some indicator in the ui for when you can throw a fireball and for when you can place a tower. Either put a number on your stats or have a line that indicates the minimum amount for each.
A way to skip the opening monologue. It's really long and you have to listen to it everytime you open the game.
A pause menu.