Good question. I live on the ground floor. The landlord had no problem with me putting bars on the windows. Very decorative so it doesn't look like a prison. And they can be opened from the inside for fire exit.

A cop told me of a good method to strengthen doors. First, you need a good door to begin with (and stron bolt lock). In a bad neighborhood, that probably means a steel clad one. When you install it, don't just nail or screw the door frame to the studs. Instead, weld one end of a sturdy steel cable to the door frame. Drill through two or three studs. Then thread the cable through the studs and attach it to a stud two or three down from the door. Do this on both sides. If someone tries to batter the door down the stress is being transmitted to the whole wall.

The guys who serve no-knock warrants hate it when the drug dealers do that. Of course, here again you will need permission from the landlord, but I have never met a landlord who objects to someone improving his/her property.

Also, have a dog. Even an ankle biter. They are great at early warning. Our apartment is right next to the front door. The dog has learned who the tenants are and only barks when a stranger comes in. And he starts barking when they come through the front gate. I know someone is coming before they even get to the front door.