At noon I scaled along the house-side as far as The coal-house door. Once I looked up— Through the brunt wind that dented the balls of my eyes The tent of the hills drummed and strained its guyrope,
Suddenly, things get personal. The poem's speaker isn't just describing something that happened somewhere to someone—he's talking about something that happened to him. He was right there in the middle of all this crazy wind. (If the speaker is a "he," that is. Since the poem strongly seems like it could be Ted Hughes describing something he personally witnessed, we'll stick with "he" for convenience's sake. But be mindful that the speaker could just as easily be female.)
He's cautious as he approaches the wind, almost sneaking along the side of the house—scaling it as though he were on some perilous adventure. He nears the door of the coal-house, where the coal to heat the house is stored.
When he looks up, the brunt force of the wind ("brunt" refers to an intense force or shock) dents his eyeballs—or it feels like it does, anyway.
The power of the wind makes the hills seem like a tent. The totally solid landscape seems like it could just flap up and be blown away. (BTW, a guyrope is a rope that helps secure a physical structure—like a tent—to the ground.)