The most vulnerable part of your flat is likely to be the front door. 

If it's weak, replace it. It should be as strong as the main entry door. Other precautions to improve security include: 

• Fit hinge bolts which stop the door being pulled off its hinges. 
• Fit a steel strip to the door frame to strengthen it. 
• Consider having a door telephone entry system installed. 
• Never "buzz" open the door for strangers or hold the door open for someone who is arriving as you are leaving.