I have 2 blog posts on security and such regarding the file system approach. Take a look:
https://www.kehrendev.com/blog/brenden-kehren/may-2015/secure-your-kentico-site,-content-and-objects-from
https://www.kehrendev.com/blog/brenden-kehren/june-2017/security-and-folder-permissions-in-kentico
Also if you want to get to a more granular approach, you might want to use web.config in each of the directories you want to restrict or grant access to. For instance, take a look in the /CMSPages directory, there is a web.config file in there which grants and denies access to specific files within that directory. I recommend creating a web.confg for the directories you want to restrict or grant access to. Keep in mind if you get too restrictive, the CMS won't properly function.