Each breed has its specific character developed along the time. All Mountain Dogs are used to stay and guard the flock, even when the shepherd went to village. That gives them a typical independent character and the capacity to act by themselves in case of danger, and all other qualities of a good watching dog.
Just this independent character makes the Estrela Mountain Dog very well adapted for our modern time, because he can wait patiently for the return of his master (and family) at the end of the day and always watching and defending house and garden against thieves.
But you can be sure: the behaviour of the dog depends also very much about the education and socialisation he has received and the relationship with his master.
The beauty, his high intelligence, the ability to observe, his vigour, makes this breed more than a simply watchdog. He is also a very a good family dog.
The Estrela is a good friend to the kids. He's strongly attracted to them and allows them to do things he wouldn't tolerate from adults.
He's extremely patient with them, playful and very protective.
It is in his nature to be a guarding dog and he begins to show that instinct precociously, when he is very young

Although he is an extremely alert guard dog it is not pathologically aggressive but will demonstrate this alertness with a loud and threatening barking. To owners he is loyal, affectionate. He knows his family's friends and doesn't forget them. With guests, previously presented, he tolerates them keeping, in general, a certain distance...

Outside of his territory he is calm and self-secure.
The Estrela is inclined to be stubborn so you will need to show the same degree of stubbornness as your dog during the training period. They have a good capacity for learning... you just have to be firm. Patience is the key to training...