added mininum to cap the chance at 90%
The attack value of a zombie unit is the number of hidden [[zombies]] under it. The defense of the [[building]] they attack is calculated based on how many police stations and survivors on [[guard duty]] there are nearby. If there are more zombies than defense, there is a chance they will break through the walls. This chance is higher if there are a many more zombies than defense. The algorithm is:
:<code>(ZOMBIES/DEFENSE) ^ 0.5 *
50 - 50
: If 10 zombies attack a 10 defense building, there is a 0% chance they'll break through.
: If 20 zombies attack a 10 defense building, there's a 21% chance.