Very cool and polished!
I had no idea that there was only 1 unit and the others was mask so I spent some good time trying to place masks on empty tiles :(
After I figured that out. I had no idea what each mask does, so I kinda placed them at random trying to figure it out... To no success. The masks need explanation or they need much clearer feedback on what they do and how they change the unit or the surrounding units, to the ones that have area effects. I ended up not beating the game because of that, I failed some times and gave up.
If I could invest more time in the project, I would unlock the masks gradually so I have a more cadenced learning of the mechanics and have simple tooltip over the masks so I can read their effects.
Aside from that, great project and great game!