Good work! I think this concept has a lot of potential.
I would like to see the enemies doing something specific, and maybe a rock, paper, scissors style mechanic with your unit types to make it more interesting. Elemental types would work too, anything to add some more strategy to the game, really. Currently I felt like it boiled down to just guessing until you got the proper balance of attackers vs repairers, and that is a bit bland.