Honestly, it would be a lot better if the zombies would actually CHASE the people instead of wandering around aimlessly while getting shot at. It doesn't matter how many upgrades you buy, the ONLY way to win is getting lucky. You just have to hope to god that the last person in a 120 person city crosses paths with one of the 50 zombies going in random directions before it's too late.
So I just bought a Legendary Lure for over 20,000 gold, thought it would summon an awesome creature to help my party, but it turns out it's AGAINST you. So basically, I paid 20,000 gold to get my ass kicked by a Minotaur with 2,200+ health.