I liked it, difficult is fine by me. But maybe build it up a little. Spawn meteors a little further away at the start at lower speed and gradually bring them closer and/or faster. And if you spawn them in the camera view, let the player now (with a flash or whatever) that he can expect an enemy spawning over there.
Earlier mentioned by others is that the spawning of a portal not always works. I thought maybe it is because you are clicking on a meteor, should check that.
Overall the game is fine, needs some tuning and graphics and a tutorial is always nice to have :)