The first line of defense at our house is a set of clearly visible security cameras, which can be seen from the street day or night. (My wife asked, when I installed them, if I was going to do a better job concealing the wires. I said, "Hell, no!")
A few days after I mounted the cameras, a pair of daytime burglars came through our neighborhood, hitting every house without cars in the driveway... except ours.
The second line of defense is locked doors. Every door on our house is locked with a double-cylinder deadbolt all the time, whether we are home or not. (We keep keys near the door, but not in the locks: we can get out in an emergency, but nobody can reach the keys through a broken window).
If the doorbell rings, I answer it with my Safepacker in my hand: it holds a Glock 29, and a spare magazine. If I am not home, nobody answers it. Both my wife and my mother-in-law know that they must not answer the door unarmed, day or night... while they know how to shoot, they simply choose not to respond to an unexpected knock or doorbell.
While we hope that those defenses will prove adequate, we are prepared in case they do not. Each of us has a firearm in our nightstand: the women have revolvers, and I have a Glock 20. Behind my bedroom door is a Saiga-12 with nine rounds of #00 Buck. (Next to the Saiga is a bag with five more loaded eight-round magazines). In the corner behind where my wife sleeps is a Mossberg 500 holding five one-ounce slugs. We have already planned what each of us will and will not do if I need to investigate a noise or disturbance in the house.
As you can see, I am a pessimist. I assume that when something goes wrong, it will go massively wrong. I intend for the women to survive when that happens, and I hope to join them.