Having completed the game, the one thing I'd like is a bit of a grace period after activating the final tower.
A bunch of enemies spawned on top of me and started walking towards the portal. Before that, the enemies had managed to spawn and reach the portal before I got to it.
I think I completed the game after activating the towers 4 times.
There could be an arrow pointing to the portal or a 15-30 second grace period to help you reach it in time.
It would also be nice if the game told you what happened when you failed.
I got careless with clearing out enemies, and they touched the portal, and then the screen goes back to how it was before. Perhaps the game could tell you what you did wrong when you fail? So you know that it's giving you a chance to try again, and the portal getting touched isn't game over.
My favorite part is that the upgrades all feel like logical progressions. It matches your other game. We can assume that Elves simply gain power when they slay enough enemies and collect experience/gems.
And this allows them to evolve in a way. Everything the elf heroine does in the game is an expansion of something she can already do.
It gives you room to add new characters and make all of them actually play differently. Someone doesn't just choose a character that has a good bonus and then play the same build.
They pick a character class. You've managed to take the Survivors genre and do something different.