The speaker of "The Splendour Falls on Castle Walls" is walking in the hills above a castle—alone. Does that bum him out? Not one little bit. He watches the sunset, takes in the beautiful scenery, and enjoys the solitude. Suddenly, though, he hears a bugle horn that seems to come out of nowhere. He's not as solitary as he thought he was! But the combination of his apparent isolation, with the reminder of other people (that he can't see), contribute to the kind of deep thinking and reflection that he's able to accomplish here.
Although the speaker of "The Splendour Falls" seems to be completely alone, the sound of the bugles reminds him abruptly that solitude is only ever an illusion. Every corner of the earth, even if uninhabited today, is still inhabited by the "echoes" of historical men and women. You can't get away!
Bugler? What bugler? The speaker of the poem addresses the bugle, instead of the person playing it, in order to sustain the illusion of solitude and of being isolated from other people.