I have one of the little miniature cameras over my front door.
If the doorbell rings, I click the tv remote to the camera.
I can see my entire front porch on the tv, so I will know who
is at my door. If I don't know them, I either answer the door
armed, or I don't answer the door.
My back door? If they knock on that one, they have climbed
the fence, because I keep both gates locked from the inside.
A knock on my back door would bring about an immediate
"condition red" full alert, with weapons being 'at-the-ready'.