Danger level one: UAV scan of the city
Danger level two: Sniper attacking the city
Danger level three: Attack-Chopper sweeping the city
Danger level four: Tank sweeps city streets
Danger level five: Soldiers sweep the streets and sewers
Danger level six: Chlorine/Poison gas shell barrage
Danger level seven: Artillery barrage
Danger level eight: All of the above
Danger level nine: Gates unlocked, giving zombies a path through to the safe zone
Danger level ten: All of the above, but, permanently destroying or damaging a building.
Level ten has been reached before. Thats why Knaztek has a giant hole in the top. it use to be a survivor base, one of the safest too.