Interesting idea for a game!
To make it even better, I'd suggest to have some words instead of the whole alphabet in the level. This way you could even add some lore and make the actions more "meaningful".
I myself don't know the position of all the letters on my keyboard by heart, so I ended up dividing my attention between looking at the screen (with the enemies approaching fast) and looking at my keyboard, which made the game harder than intended, I suppose..
Great game! Congratulations!