Watchdog and guard dog are two different jobs
A Boston Terrier can do the first of these jobs surprisingly well. Watchdog work is detection and announcement: hearing the gate, the footsteps on the path, the unfamiliar car, and telling the household about it promptly. Bostons are alert, people-focused dogs who keep track of their home's normal sounds, and most will bark readily at a change in the pattern. In a flat or a terrace, that early warning is genuinely useful — you will know about a delivery, a visitor or a prowler at the fence sooner than you would without him.
What happens after the alarm goes off
The Boston's limits appear the moment the door opens. This is a small, friendly companion breed with no instinct to confront or detain anyone; the stranger he barked at thirty seconds ago is now a person who might play with him, and he will usually act accordingly. His bark is also bigger in confidence than in volume, and his size deters nobody who has actually seen him. Anyone choosing a dog primarily for protection or deterrence should choose a different breed entirely, because expecting guard behaviour from a Boston is unfair to the dog and unreliable for the owner.
Too much of a good alarm
The watchdog trait needs managing in a breed that enjoys having something to report. A Boston left to practise barking at every passer-by from a front window will refine the hobby, annoying neighbours and keeping himself in a state of arousal that spills into the rest of the day. Teach a clear pattern instead: let him bark a few times, thank him, then call him away to a treat or a mat so reporting ends with you taking over. Blocking a favourite lookout spot when you are out, and giving him a walk and a game before work, lowers the number of events he feels the need to cover.
Flats, terraces and the neighbour question
Where you live decides how much watchdog you actually want. In a detached house, a Boston's door barking is a private matter and usually an asset. In a flat with thin walls, a dog who announces every footstep in the shared hallway will generate complaints long before he generates security, so training the thank-you-and-settle routine is not optional in that setting. Be honest about that before choosing: the trait itself will not be trained away, only channelled, and a household that needs near-silence should weigh a quieter breed rather than spend years fighting a companion dog's nature.
Judging the individual dog you are offered
Watchdog behaviour varies by individual more than by breed average. Some adult Bostons bark only at the doorbell; others narrate the street. When you adopt, ask the previous home or rescue for specifics — what sets him off, how long he continues, whether he settles once a visitor is inside — because those details predict your daily life far better than a breed summary does. Those specifics are worth keeping with the dog himself: through KinsLe Transferrable Profiles, what his last home learned about his alerting, settling and household routines transfers with his profile, so his watchdog habits arrive documented rather than as a surprise on your first evening.