Zombie attacks are not random, I've beaten hard and nightmare and its all about having soldiers clearing out the zombies around you 24/7, By having a larger "base" you have a larger area for where zombies to attack you. So I consistently send out 1 soldiers to all areas around my base to keep "clean"
This game is good but it does not accept other "correct answers" and requires you to move the objects to other areas. For example, moving a block from right side to left even though it wills till look exactly the same.