I would check for typical problems first: Cyrillic characters in the map path and an excessively long path. If this is the case, the map will be corrupted and will weigh only a few bytes. You can restore it by packing the files in the Editor/Maps/map name folder (if I haven't forgotten, otherwise I might get confused).
Regarding the rest, try it out, and if you have any questions, ask. It's quite easy with cities, but you can also unlock heroes; as an example, you can look at how mods like NHF and others work, which add heroes or remove the lock on them.