A cylindrical deadlock on all your external doors (or a cylindrical lock set on a home office or bedroom door) gives a sense of security that more than compensates for the effort and cost involved in its installation. You will need to bore two holes and do a little chiselling to install a lock of this kind. Mortice locks, on the other hand, need many more holes bored.
A deadlock ensures that, even if an entruder gains access via a window, he or she can't open the door. A deadlock has no handle and can only be opened with a key (inside and out) and the idea of having to haul your heavy valuables through a window may well see the would-be burglar on his way. Keyed locks on the windows are worth considering.
A safety chain on a timber front or back door allows you to open the door marginally to see who's calling, but won't afford an opening sufficient for anyone to enter. It also doubles the security of a lock and makes you feel a great deal more comfortable about being home alone.