I added a Level to a user, but it can still see other content?
When you create an Access Level to restrict some content, only users with this level will be able to see that content, but they will also be able to see all other (unrestricted) content on your site.
A quick way to “lock down” and make sure e.g. only Administrators can see all content is to create a new Access Level for Administrators with a negated condition group containing “404 Page”. This means that normal users only can see the 404 Page.
By default, Administrators will have access to all content regardless of your levels.