I had the same issue with the menu not going away, but I used the scroll wheel like you suggested and it worked!

I couldn't tell what the different terrain types and weapons did.

Actually, no matter how I placed my weapons I didn't ever see them fire at the monsters so I'm not sure if they work.

Eventually, I figured out how left mouse button place terrain and right mouse button picks up terrain.

I used this to pick up a bunch of lava, then I surrounded my crystal with lava and it totally thwarted the monsters!  They would just spawn and sit in the center square and not move.   I guess maybe just have them move around randomly if they can't find a proper path to the crystal?

There's definitely the kernel of something really fun here, I encourage you to keep at it and keep us posted on your progress!

I wasn't able to play this, due to all of the reasons Ben mentioned. I don't think this is a hardware specific issue, either. I've tried to play it on two very different setups (One: Intel + NVidia, Two: AMD + Radeon) to no avail. Don't be discouraged, though! First game jam ever and 13 years old? Yeah man, you're off to a great start!