Greetings, I follow the series since the release of the first one :) I think it would be better to have an aditional key in order to start the wave. You have your one hand ont the 123qwe and the other on mouse, so you need to move one of your hands in order to start faster the next wave. My solution is to use my thumb from the right hand to press the NUMpad's enter :P It's not the most comfortable thing i've done :)
Grats for the ideas and the game.
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There will be added an extra key near the other hotkeys in the update!