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.