How a mountain road may rise or fall

STEEPLY

Think of how a cliff’s slope might be described in just one sharp word.

This clue describes the way a mountain road might sharply ascend or descend. "Steeply" fits because it directly means having a severe incline or decline, which is common on winding mountain paths. The word itself is straightforward—no hidden wordplay—just a literal description of how such roads behave.