Fit insurance company approved locks. If you have to use a key to get out , keep it nearby - just in case you have to get out in the case of fire. Never give others information about yourself or your life style. No matter how intimate - a stranger is still a stranger. Never give any indication that you are alone.
Some people just love to look through windows.
Make it uninteresting by drawing the curtains. If you get worried, ring law enforcement Keep unknown or unexpected visitors out. Use a door peep hole, or use a window. If you can't see anyone, do NOT open the door. If you put a chain on the door ensure that both the chain and the fastenings will withstand a kick.