。
The road zigzagged up the steep slope.
Literal
Road [topic-は] steep slope [object-を] zigzag [to-に] climbed was.
急斜面 (きゅうしゃめん, 'steep slope') is a geography/terrain compound. ジグザグに ('in a zigzag pattern') is the English loan used adverbially. のぼっていた is the past progressive of 登る ('to climb') — describing the road's path as if it were alive. A vivid landscape description that personifies the road.