A small guide for later levels if your stuck
10/10 Loved it. It could do with a storyline and perhaps a quicker way to get those monies coming in but still a pretty good game.
I finished all 5 levels. My planning was to get to that maximum Salvage Yards as soon as possible to bring out the bigger units. With later levels you should wait to spend your first 500 on that first yard. Then when get the zombies get too close, save up 250 even if they are bashing at your base. Quickly produce 5 shovelmen simultaneously as these guys work better in groups. Use these 5 to push the zombies back and save up for that next 500. After this you may need to produce 2 groups of shovelmen and then save up that extra 500 because those witches will tear through them.
Once you have maximum salvage yards. Keep the zombies at bay with small groups of 5 shovelmen until you have saved up enough for the angry farmers. Buy one or two and another group of 5 shovelmen. Keep producing angry farmers, say 10 or so and hopefully at this point you will have enough 'Yeehaw' points to bring out an angry mob. Build trailers if your population gets too big.
Now save up for a Still and produce Moonshine bombers along with the occasion mob run to refill your anrgy farmers. Keep on producing Moonshiners and mob runs with the occasional angry farmer and their base should be finished.
I'm not sure this is useful for 'Speed Runs' but it works if you want to finish the game.