Really impressive artwork and atmosphere! Unfortunately, I couldn't quite make it through the final boss, but I did manage to get pretty close.
The use of modes was cleverly implemented, though I think the assault mode was pretty overpowered for most of the game thanks to its high number of shots and long range. I relied on it to the point that I was taken pretty off-guard whenever I had to actually change strategy.
I think that maybe shifting tiles with environmental effects might encourage more varied tactics and unit movements, since staying immobile near the center is a bit too viable for most of the waves, but that's just a spitball idea.
Overall it's an impressively polished entry!