Overall I found this game pleasant and fun to play. It's like a close-up version of plague.inc, which is fun.
I think the best way to make the game even better would be to allow the selection of some units to issue one command and others for a different command. This would enable more tactical gameplay and open up a lot of options for further difficulty scaling.
My only complaint is that the zombies would auto-target a house even when I was trying to get them to target a soldier; I think the auto-targeting works pretty well and is a good thing to have in the game, but if possible it would be great if it slightly favored humans over buildings instead of just proximity.
Really nice work!