Very cool concept, I like it!
I think the difficulty from the start is too much I don't mind the game getting hard quickly but pace it out, I think making the tower cooldown ticking while it is not selected would be easier so that I can switch tower and it shoots immediately. Maybe make the first upgrade almost free (like kill 2 ennemies and get an upgrade) but make the cost go up for all of them at once, so I can spec in one or the other at the start?
I really like the design, I get some minimalistic mini metro vibes, perfectly fits the game :-)