Get in the habit of ALWAYS using Peepholes before answering doorbells or knocks. Why not? It only takes a few seconds. Or would you rather gamble? Gamble in Vegas - not in your life.
You can supplement a peephole with a doorbell intercom described at Home Intercom System.
Also strengthen your door's hinges – preferably the entire door and doorframe. See Door Reinforcement.
There are FIVE primary ways of invading your home:
My #1 Pet Peeve is the Walk-In. Doors have locks for a reason: to keep out anyone you didn’t invite in. Roughly HALF of the 7,000+ home invasions every day in the U.S. alone are through unlocked doors. I can never understand the mentality of folks who are so careless. There’s NO excuse for not keeping your exterior doors locked at all times. See Walk In for more details.
My #2 Pet Peeve is the Push-In. It’s the second most common method of home invasion – and almost as easy for a criminal predator as the Walk-In. Again, I can never understand the mentality of folks who are so careless. How can you possibly just assume that whoever happens to knock on your door or rings your doorbell is not a criminal predator? See Push In for more details.
Closely related to a Push-In is the Scam-In. That’s when a criminal predator uses lies to get you to open your door and/or allow them to enter your home. See Scam In for more details.
The final two methods of invading a home are:
Also see Home Security – Cracks for variations of the above.
It's your castle - defend it.