Current questions and answers regarding the map editor.

To understand which Hole tiles need to be removed, it is enough to know the location of the center. Finding the center is very simple: in the Objects Properties Tree, find Pos => + => set any coordinate, press Enter. Then, look at the city from above, hovering over the cell with the same coordinates: the cell located at the coordinates set manually through Pos is the central one.
It is easier to remove all Hole Tiles and then add them back if needed, or to create a new city instead of modifying an old one.