When we create a project there is a default or a defined folder that is used. But sometimes we use a template project and keep not this project but the folder is created and after we have a lot of folder without nothing inside … is there a elegant way to delete the folder if the project is not used or is it possible only with windows file explorer ?
Deleting is possible In the Finder/file explorer only. But, why to delete it?
You know, there is Audio folder, for example. This looks empty, when you quit the Cubase. But if the Cubase runs, this folder is used for buffering of audio. So, it has sense.
yes but sometimes we don’t think to create a new folder for a project and after we don’t know where are the files …
it’s a nightmare to make a clean project with all in the same folder after saved.
That’s require a rigorous methodology 
In this case, it is best way to make a backup file. You can find this in the File menu. Douring this process, you will choose new folder for the project.
Or you can use Open > Recent projects, and check the Path in the menu. Or you can use Spotlight (or Finding) in your operating system.
However, in default, Cubase saves projects to the “Cubase Project” folder. This is placed in the user/Music/Cubase Project. I suppose, on Windows, it will be very similar.
I have a song folder where there are all my projects, this is important a good administration of the folder specially for the archive process. Lucky there are some functions in cubase to archive a clean project and to win space on the HDD by deleting unused files or retrieve missing files.
Yes, Backup project is one of them, which is very useful!