Current questions and answers regarding the map editor.

you can use it as a model for a ruined city, for example, a log or a stone. But it won't be possible to position a hero on the former outline of the city (a circle with a radius of 6).
If the methods mentioned above don't work, then I don't know any more.

It will work when the destruction of the city actually changes one object (a city with blocked tiles, which is what you were talking about) to another, where the tiles don't need to be blocked. If there is no model in the new object, and the tiles are not defined, then after the city is destroyed, the game will leave an empty space where the hero can move.