I like the update. Have you considered traps as a mechanic? Reusable, resettable traps you can plop down? Could be a nice alternative to the player spending their gold on a tower.
Pit traps, bombs, glue traps. Maybe walking over one would reset it? It's a more active mechanic than the set and forget of a tower, but the traps would be consistent if you can have the enemies step into one.
The other thing about the tower system is that you could assign a feature to them that disables enemy spawns near them.
The main difficulty in this game is of learning where you need to stand. If you stand in the same place, the enemies will spawn in a predictable manner. If you move around a lot, enemies can spawn directly on top of your towers, or right in front of them inside your lines.
They're programmed to appear outside of your line of sight, so you can get weirdness if you run out to collect points and then walk back to find they appeared and wrecked all of your guys.
Could it be a thing that they all appear from the edge of the map and then gradually move towards your position? Then the defense aspect would always be consistent.
You could mix in some flying enemies who can soar over obstacles to keep up a steady amount of pressure on the player.
And add a climbing feature to some enemies. I think it's really cool how they can get stuck on things, but you wouldn't have to worry about pathfinding as much if they could group up and then climb over each other's heads to get over something.
As far as the power ups go, I think they're balanced. I personally like the one that lowers your spread. Being able to effectively snipe enemies and make a wide stream of arrows hit where you want them to be a highly useful tool.
And it apparently upgrades your speed, too? I assume this is the speed that the arrow travels? It doesn't seem to increase firing speed like the shoot power up does.
The only one that feels like an oddball to me is ice, because the others all have an effect that kills. Fire creates massive explosions. Acid was so powerful you had to make it work a limited number of times.
And then we've got ice. Perhaps it could apply a frost bite effect that makes enemies unable to attack until they warm up? That's something an ice specialist would love to have. It means they have more chances to protect the portal. The guys can't hurt it if they're still chilled and are blue.
Thanks again for the update. This is my most anticipated game, and I'm always happy to play it.