Great little game, thank you! 3 points:
1) At the bottom of the screen, 'Hold your leftt mouse button' - 'left' has one 't'.
2) You have lots of ways to speed it up, but none to slow it down (so that you get less enemies added per second, and more time to handle the ones that come up). Yes, you can just not update rotate and walk speed, and not speed up its walk, but still, people might upgrade beyond their abilities and regret it... Holding the key in the opposite direction to travel should slow it down, eg holding D while going left should make it slower.
3) You may want to make sure that the click helpers go in different directions, to different targets, rather than as a group - you could just hold a list of enemies already targeted and ignore those ones (or a list of those not targeted, either way).