Do I need to do anything special to secure the hierarchy?
Not really. Since the hierarchy is showing information about your related records through a portal, it will respect any security privileges you have in place in your data tables.
However, you may wish to use FileMaker's build in Access Privileges to prohibit creating and deleting new records in the Hierarchy table for most of your users. If you're new to this, open FileMaker's built-in help and browse for:
Contents > Protecting databases with accounts and privilege sets > Creating and managing privilege sets > Editing record access privileges