Sonnet 2 opens with a metaphor that compares the way time wears away a person's face to the way an army attacks a castle. It used to be that if you were holed up in your castle, you were pretty much safe, since any army that tried to attack would get creamed when it got close to your castle walls. However, the attackers could choose to just surround your castle, close off your escape, and wait until you used up all your supplies and got hungry enough to give up. That kind of wait-and-see attack is called a siege.
- Line 1: Here the "forty winters" give us a strong image of how cold and merciless time is, waiting like an army at a castle wall, which Shakespeare cleverly compares to the young man's forehead ("brow"). Even though the "wall" of his face is strong now, we know who is going to win this battle.
- Line 2: Now that you have that first image in your head, Shakespeare starts to play with it a little. In the second line, he continues the extended metaphor of the siege. If that army is going to wait for forty years, they need to make themselves comfortable, so they have to dig a few trenches around the castle to protect themselves. These trenches are meant to stand for the wrinkles that come with old age, that carve themselves into our skin, which is our "beauty's field" (line 2). This is a nice bit of hyperbole (a poetic word meaning "exaggeration"). Wrinkles aren't as deep and jagged as a trench, but when you look at them closely in the mirror, and start to freak out, maybe they seem that way. One more thing: when you give human qualities to something like time, that's called personification. So, when Shakespeare compares the passing years to an army, he's personifying them. Of course, time doesn't have a personality or a brain, but in this poem it becomes a dramatic character with its own motivations.