This game's spritework is very nice, and the different music indicating my current enemy type is an inspired choice, I don't think I've played another game this jam that's done too many interesting things with the soundtrack so far, I love it!
I don't have too much to critique, but I think the controls of the characters feel a little bit sluggish. I found myself mashing keys to try and move my army around, rather than feeling like I was actually controlling them. The monsters attacking on a consistent rhythm also took a little while for me to figure out, as there's a lot of different mechanics to this game with plenty of moving parts, and it felt pretty overwhelming to try to learn everything all at once. My personal backseat design suggestion would be to maybe make the tutorial its own level, with just one enemy type? The ramp up from level 1 to 2 was also a huge spike in complexity that took a while for my brain to process all at once.
That all said, I think the core concept of the game is super inventive! The strategy of positioning all my units at once forced me to get really creative with my army's positioning, and you could definitely take advantage of this with even stricter level design. With a few more levels, alongside some tweaks to the level balance and overall feel, this could be fleshed out into a full title.