Neapolitan Mastiff

Hey all the dog lovers out there! I've just bought a Neapolitan mastiff for my home, it is a 13 months old dog. I'm a resident of detroit, USA and remains in the office for most part of the day and in recent times house invasions have increased manifold, therefore I have bought this dog for the purpose of watchdog and also as a companion for me as I stay alone. I want to know whether this dog will be able to handle the responsibilities of a watchdog or not and also if it is going to prove a good companion for me or not? Thanks!