Thank you for your detailed observations and review! <--- I hope that my comments are helpful.
I think the reason that it might be a bit hard is that I built the game for people who already know tower games like that, to have a challenge. <--- Yeah, I am not a great gamer of this type of game.
I didn't add an intro tutorial. <--- If you plan to release a commercial version of the game, a tutorial will really be essential.
Here's a tip for you to win: <--- Very useful hint for me to play this game easily in an enjoyable way. No doubt! Thank you.