Mediocre.
I'll give you a list of my pros and cons:
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Pros
Rewards
Medieval music
Nice animation
Statistics of your gameplay
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Cons
Not many towers/buildings to choose from
Only one straight road to the castle (atleast for me, because my game freezes at 8 minutes and 13 seconds, so I am only able to play level 1, dunno if other people experiences the same thing as I am. I want to test the other 2 levels aswell, I would appreciate if you could fix it :3)
Poor gameplay
Too easy, was it neccesary to change the difficulty?
I dont think that 3 levels are enough for a game
Could've been more gadgets, for example when a tower reaches a certain level it would get a special ability, but this is only neccesary if you wont make any more towers and buildings, just to balance out the monotonous gameplay.
My tip for you is to check out some other tower games to get ideas and tips to help you improve your own game.
Now I'm just waiting for a sequel or maybe a brand new tower-game from you, keep up the good work! :)