When Apocalypse comes:

• Gates and tunnels of all domains get opened and can no longer be closed,
• All restrictions on attacks due to Ranks are nullified,
• Ruins get more and more,
• Ruins appear with greater amount of monsters,
• Monsters in Ruins breed faster,
• Monsters pillage more often,
• After Apocalypse has started, monster no longer attack with «Robbery» mission, but only use missions «Buildings destruction» or «City destruction»..
  

If there are any (even damaged) Defensive Buildings in the city, the ruin monsters attack using the mission «Buildings destruction». The type of target building, no matter already built or being built, is determined in the very moment of launching the attack.
If any defensive buildings are being constructed in the city, the ruin monster may opt either for «City destruction» mission, or for «Buildings destruction» mission in order to destruct not finished defensive buildings.
Are there none defensive buildings in the city, the ruin monsters strike with the «City destruction» mission.