Thou hast made of a city an heap. No particular city is pointed at. The prophet has in his mind the fate of all those cities which have been enemies of Jehovah and persecutors of the saints upon earth.
A defended city; i.e. "a fenced, or fortified, city." A palace of strangers. As the "city" of this passage is not an individual city, so the "palace" is not an individual palace. All the palaces of those who were "strangers" to God and his covenant have ceased to be—they are whelmed in the general destruction (see Isaiah 24:20).
They will never rise again out of their ruins.