This was really refreshing to play. depending on how much time and effort you want to put into it this could really end up as a game i'd even be willing to buy, reminds me a lot of Buggos on steam.
Now for the feedback/suggestion part:
Art:
-i feel like there's an identity crisis (probably because of free assets you took here and there) you can either choose to go full on fantasy which in my opinion would be bland and soulless or you could just go all out with the starcraft flash game aesthetic.
-add more tileset, weather shouldn't be this high up in your roadmap since it risks adding visual bloat.
Gameplay:
-you should add a cursor and hotckeys for skills and clicking a skill should automatically deselect building construction.
-Skills should be moved to the Skill tree along with the different buildings you unlock automatically would make progression a bit better, make an evolution chamber building that can research the speed/health/damage upgrade to reduce bloat in skill tree.
-Units should be able to pass over buildings but be included in the grid (you can only have one unit in a square) this would stop the massive deathballs from forming and make higher quality units more valuable, for further update it'll also probably make it easier to program ranged unit since you'll be able to calculate range by block distance and air units would be valuable because they could stack one time on top of your ground units.
-Creep spread needs some tweaking to stop it from removing large patches of it when one of your building dies despite having other buildings around it.
-Add more elevations level that future ranged units and defenses can take advantage of.
-Consider some roguelike elements to make it replayable.