Seems like you already have a decent foundation for a tower defense. I tried it on web and Windows. The web version definitely has some oddities (which you seem aware of), but I didn't experience any issues with the Windows version.

Tutorial:
- I think it's okay, definitely would be better if it was more reactive. My only issue with it was having me try to build a tower and then telling me I need money to do it. I think the concept is common enough that you can just have the player pick up the money and immediately build the tower.
- I don't know how much faith I'd put in playtester's claiming that Esc for pause menu isn't intuitive, that's been a staple of game design for 30+ years. Maybe for someone unfamiliar with PC gaming that would be odd, but it's the industry standard, so don't waste time worrying about that.

Feedback:
- The main menu was not clear, in my opinion, as I didn't realize there were game modes other than Tutorial on my first 2 times loading the game.
- The flying fish on the last round of story mode taking a different route felt a bit unfair. I don't think there was any indication that enemies could come from a different spawn, and I didn't have sufficient defense or money to stop all of them.
- I do think the theming is a little odd at this point, having an arrow and cannon tower next to a missile launcher feels a little odd. Nothing wrong with it this early on, but I'd like to see unique, thematic towers as the game progresses.