To make it accessible to logged in users only, make the category only accessible to TL0 users. Since every logged in user is a TL0, this can be done.
See, when you are a TL1, you are both a TL0 and a TL1. When a TL3, you are a TL2, 1 and 0. Therefore, making categories only accessible by TL0s allow topics in there to be private.