Cool game ! A bit hard to pick up at first, though well balanced once you get it.
The wave was a bit long & hard for me. It might be a cool idea to start with a shorter & easier one to let the player get familiar with the gameplay before throwing a challenge at him.
Small bug : it seems that sold coal burner stay active on the map as the ground stay unfroze & keep using coal.
Appart from that, this is an impressive amount of work for a week & the hexgrid is a very nice choice for that genre. Great work :)