Why flattening a city can strengthen its defenses

What effect would falling skyscrapers have on urban fighting in Taiwan?

They would create rubble and ambush terrain that makes movement and assault far more difficult for the invading force.

The example assumes the skyscrapers have been evacuated after warning sirens. Their collapse would still leave large quantities of rubble and difficult terrain to cross. Fighting in an intact city is already difficult; rubble adds obstacles and numerous potential ambush positions.