Its in the ImprovementDefs xml file,,and there a a TON of indestructible buildings,,I just went through all of the buildings and deleted the line of code"<IsIndestructible>true</IsIndestructible>" for each and every one of them. I modified the megalopolis (<UpgradesFrom>City</UpgradesFrom>) and siliconcity (<UpgradesFrom>Megalopolis</UpgradesFrom>)so that should do it.