…At fourteen I became your wife, So bashful that I dared not smile, And I lowered my head toward a dark corner And would not turn to your thousand calls;
Wow, people married young back in the day, didn't they? The speaker's only fourteen when she marries her lover (who is probably her age, or not much older).
These lines paint a picture of the early days of their relationship. The speaker's so shy with her hubby that she can't even smile.
She doesn't even answer her husband's calls (and no, we're not talking about telephone calls here—she doesn't answer when her husband actually calls to her). Props to him for not giving up, though. A thousand calls? That seems a little extreme, even for folks in love. Likely, this is an example of hyperbole on the part of our speaker.
This speaker just hides her head in a "dark corner" instead of answering her husband. These lines give us a sense of just how young and shy the speaker is at this point.
But at fifteen I straightened my brows and laughed, Learning that no dust could ever seal our love,
Finally, after a year of being married to her lover, our speaker gets over her shyness (about time!).
The image of "straightening" her brows suggests that the speaker has stopped being so shy and worried. If we straighten our brows, that implies that we're not frowning anymore.
More good news: the speaker's laughing. That's some progress from being so shy that she has to hide her face in a dark corner.
The line "Learning that no dust could ever seal our love" is ambiguous. Dust evokes burial and death. So the speaker could mean that death can't "seal," or close off, their love. That is, death can't destroy their love.
What we see in these lines is that the speaker's really grown into her relationship with her hubby. This is one in-love bride.
That even unto death I would await you by my post And would never lose heart in the tower of silent watching.
The speaker's so in love with her hubby that, now she's gotten over her shyness, she would be willing to wait "unto death" by her "post" for him. What's a post? That's a place where someone stands to keep watch for someone else.
She tells us that she would never lose patience, or "heart," in the "tower of silent watching." The image of the "tower of silent watching" gives us a view of the speaker looking out of a tower (kind of like the way that a watchman looks out of a lighthouse), in silence, waiting for her hubby. It seems like this guy goes on trips every once in a while, though the speaker doesn't tell us why. Maybe he's some sort of ancient businessman?