I pressed H for the tutorial on level three and got this error. Seems like an index error, hope this helps!
# FATAL ERROR in; action number 1; of Draw Event; for object oMain:
Push :: Execution Error - Variable Index [0,8] out of range [1,8] - -1.tut_txt(100196,8)
at gml_Script_tutorial_execut #
stack frame is
gml_Script_tutorial_execut (line -1)
gml_Object_oMain_Draw_0
The game is a great concept. I think the difficulty scales a bit steep, but otherwise, really really cool start. I think maybe a good way to assist in this is to at least start with the $15 torture rack. It makes it tough to get towers in strategic places when I have to save some money to earn money (which is a great idea btw, just tough to start levels). You could even Extend that idea into difficulty. Easy mode you start with 3, hard mode you start with 0.
The only other real issue I had was inputs being clear. Evengy mentioned some of the issues, but I think that since a tower defense game is mostly mouse controls, I think having at least left click being an action button, and maybe a button in some places should help with that issue.
I also didn't know that there were any upgrades until reading evengy's message. Maybe a more in depth tutorial for each mechanic might be helpful for this.
Otherwise, and I cannot stress this enough, this is a really really great start. The art is so great, and the mechanics are so smart. It's hard to find good tower defense games, and this is a great tower defense game.