A lock on a flimsy door is about as effective as locking your car door but leaving the window down. It should not have a window set into it as a thieve can break the window, reach in, and unlock it from the inside. If your doors don't fit tightly in their frames, install weather stripping around them. Install a peephole or wide angle viewer in all entry doors so you can see who is outside without opening the door. Remember, if in doubt, keep them out.
Door chains are false security .
They break easily and don't keep out intruders. Never leave notes on your front door. Remember, any door is only as strong as it's weakest link. Pay special attention to back doors and windows, where burglars often find the easiest and most secluded entry. Check the outside Look at your house from the outside as a burglar would.