While the speaker of "The Castle" never formally introduces himself, the poem's opening stanza makes it pretty clear that he's a soldier defending a castle against an enemy army. That army doesn't seem to be much of a threat in the speaker's eyes, however, and the poem begins by establishing the calm, leisurely, almost pastoral atmosphere surrounding this mighty castle.
"All through that summer at ease we lay," the speaker says, adding that every day from "the turret wall" (or the little towers at the top of a castle), the soldiers "watched the mowers in the hay" down below. The image of soldiers lounging about watching people mowing in the fields suggests an awfully relaxed (and even complacent) scene considering the enemy army is stationed only "half a mile away."
The word "that," however, hints to readers that this "summer" will be an important one: it's not just any summer, but "that" specific summer when something big happened.
The form and sounds of the poem add to its calm, quiet tone. These lines are written in iambic tetrameter, meaning each line has uses four iambs, metrical feet with an unstressed-stressed rhythm. There are some minor variations here and there, but, for the most part, the meter is steady, as in line 2:
And dai- | ly from | the tur- | ret wall
The steady ABAAB rhyme scheme also helps to infuse the poem with a sense of predictability and order.
Enjambment also helps to keep this stanza feeling open and relaxed. The lines flow smoothly down the page, evoking the soldiers' complacent comfort.