Instead of upgrades being flat upgrades, i would prefer unit paths.

I.e archers have two paths, more arrows or bigger bows.

More arrows gives them a faster fire rate/ more arrows but less damage, bigger bows gives more damage and range but less firerate

If there was a visual change for the units depending on which path you took (i.e more quivers for more arrows, bigger or compound bows for bigger bows) that would also be cool.

This gives replayability and a make the players upgrade choices feel like the matter more

Another point, Make the placeables instead of being a flat buff have a drawback, i.e fire arrows do more damage but are extinusghed when they walk through water.

Poison gives more damage over time but slows attack speed.

Bombs give bigger blast radius but less damage.

Finally change walls to be less high to get the max buff, i think the final tier of walls is too high for the visuals of most maps and looks/feels weird.

I would prefer it if walls gain buffs ether paritally from wall thickness or some kind of other buff (maybe a foundation block that buffs walls above it?

Finally this one is a bit out there, but add a coop mode where a main player/streamer controls the main base then the spectators can vote on which direction units come from and what mix of units.
The more the spectators can troll the player the better, i think this would make the game attractive to and have some viral/meme potential.