You shouldn't need such a huge cell size, in the first place. The city should take up from 0,0 to 1521,1521 coords. One change I made, which helps prevent sections of the city from being broken off, is to change the city size in the "smallCity" hub to 1600. This will add a bit of flat ground outside the city, and help mesh it to the land. Then, one can use a smaller cell size than 4200, more like 1800. Changing the cell offset in the first line to a very low value is also important, in that if you have the city pressed up against, say, the eastern side of the cell, and another hub to the east is pushed up against the western edge of its cell, the map will generate with roads and prefabs reaching into Slaanhatten, which causes all kinds of problems, I'm sure you can imagine.