Pretty good game for what is currently there right now. I don't really have any complaints about what's here right now, as the gameplay works well with the unique mechanic.
Some ideas I think would be cool, but aren't necessary would be a sort of projectile-like enemy that would fly past and tries to hit you but doesn't 1 shot you so there is more danger on levels like level 2, an enemy that instead of killing you will try to burrows and traps a key on your keyboard, and a tower that when placed lets you use your own cursor to attack (you can only place one and it would be a bit expensive).
Overall, the game's good, and there are most definitely a lot of cool visual and gameplay things you can do with this, so good job on what's here so far.