The bullmastiff is derived from breeding the English Mastiff to the bulldog (of years ago not so much the bulldog of today). This breed was created in the early 1860's by the English. Their goal was to produce a dog that was large but not overly aggressive. One that could do the job of taking down a poacher but not eating him alive. Fascinating history as they were first recognized in the kennel club of England in 1924 but not accepted into the American Kennel Club until 1933 almost 10 years later.
These dogs were bred to be strong enough to take down a grown man and hold him down. They are fearless and typically do not bark without a reason. Today the breed finds it's place in the family home. People call them "gentle giants' and they are until you see the 'guard' side of them display itself. Good with well behaved children. Often they will not tolerate unruly or abusive children.
I tell people if you want a dog your children can climb on, step on and pull their ears and tail get a lab or a golden. These are working guard dogs and they take their job seriously. Some of them will not tolerate being disrespected or having children in their face. The will often get up and move away from a situation. The bullmastiff is not like other dogs (think Husky) that threaten and make a lot of noise. If they do bother to growl to give a warning they are not bluffing. They are to be taken seriously.
This is not a breed you send out to have trained. They obey you because they respect and love you. You must go to training with them. I have only had a couple of puppy owners, against my good advice, send their bullmastiff out for 'protection' training and both parties were very disappointed. You do not want to train your bullmastiff to be aggressive toward people. It goes against their nature. These are NOT German Shepherd Dogs. Guard is as natural as breathing to them. It's what they are hard wired to do.