While I do like the game, it seems a bit fast paced you lose health very quickly which seems unmerciful with no real way of recovery.
On the contrary the enemies that you have to click do pose their own unique challenges like some being faster than other or some that move your base leaving you to deal with wherever it may take you on the next wave, or some that try to escape your cursor. Though one enemy that didn't seem all that fun to work with was a gray type that would pursue your base that would move quickly as well as being very difficult to see against the faded blue background.
I do appreciate the small secret that when you try to use the arrow keys it allows you to change the travel trajectory of some enemies and adds a level of strategy to move some out of the way of your base or bring the final enemy to a more advantageous spot to finishing a round.
A decent time killer and unlike other replies I feel the game still holds its own without upgrades or graphical revamps.