An alert, perhaps; a guard, never
Judged as a watchdog, a Cavalier King Charles Spaniel fails cheerfully. She may lift her head and bark when a parcel lands on the step or a stranger's step sounds on the path, and that brief alert is genuinely useful if you work in a back room. But a watchdog's real job begins after the alert, and there the Cavalier's nature takes over: the person at the door is, in her judgement, a new friend. She has neither the size to deter, the suspicion to hold her ground, nor the protective instinct that makes some breeds place themselves between their family and a threat. Expecting more from her is unfair to the dog and unwise for the household. She will not patrol a garden at night, investigate a strange noise with any intent to confront it, or hold a doorway against anyone; the most she offers is a brief commentary before she goes looking for a lap, and owners who buy the breed hoping training will install a guarding instinct are reliably disappointed within the first month.
Why the breed was never meant to guard
Cavaliers descend from the small spaniels kept at royal courts as companions and comforters, carried, petted and bred for sweetness around people. Nothing in that history selected for territorial behaviour, and centuries of choosing the gentlest puppies produced a dog whose default setting with strangers is delight. That is exactly why Cavaliers excel with visitors, children and care-home visits, and exactly why they cannot be trained into watchdogs without fighting the breed's deepest instincts. You can teach a Cavalier to bark on cue and to stop on cue, which sharpens the alert part, but you cannot teach her to mean it as a warning, and a determined intruder will read her wagging tail correctly in seconds.
What to arrange instead
If a dog's protective presence is a genuine requirement, this is the wrong breed, and a more naturally watchful dog should be on your list. If you love Cavaliers and simply want reasonable home security, let other systems do the guarding: solid locks, a doorbell camera and lights on timers all outperform a friendly spaniel. What a Cavalier does contribute is neighbourhood goodwill. She will know every dog walker on your street by sight and be known in return, and that web of people who recognise your dog has real value if she ever gets out, because neighbours who know her face are far more likely to catch her and bring her home.
A wider circle that watches out for her
Turn the question around and the Cavalier's friendliness becomes a form of protection for the dog herself. A breed this trusting will approach anyone, so her safety depends on other people looking out for her. Through KinsLe Circle of Love, your free account connects to nearby pet lovers in a local network, and those neighbours become extra eyes for your Cavalier: people who know she belongs with you, who would notice her trotting loose down the street and who can help when she needs people watching out for her.