Sense doors are your senses: Seeing; Hearing; Taste; Touch; Smell & Mind (The mind can be an originator of information without the other 5 senses triggering it, thus it is a sense).

To protect is to shield or guard against.

If your sense doors are protected, then they are not registering information or triggering computational thoughts. If your not registering information, you are not forced into thought labor. If you are not forced/compelled into labor, then you are free: Liberation from info-slavery.

So how do you protect your senses?

It is not the absence of stimuli, it is the absence of reaction (not to be confused with indifference or desensitization). If these reaction opportunities (sensory triggers of information) are met with consistent neutrality, then you are protected.

To strengthen this ability and introspect/insight empirically you should meditate. The “Depth” course is a simple, non-dogmatic, vehicle to do your own science experiments for the validity of this information.

At least this source of information is encouraging you to verify your own truth, representing an alternative path to all of your taxation (suffering) and freeing you from slavery… Merica.