you can to some extent use litiums build in permissions to say the you need to be part of a group to view a specific page. login page need to be open to anyone in that case.
We have used the same thing that Jonas Kastebo above says, we’ve locked down all pages so they require a group permission.
Do note: if you set permissions on startpage, and select that all subpages should have same permissions, you will break order confirmation emails or other emails. Litium calls iteself to get html email content, those pages can not be locked down
You might also run in to this: if you have a search field in the header, you will still be able to search for products for example, if you don’t have permissions set on those, or disable the search if user is not logged in etc…