The Hebrew word used here is the common word for ascending.
The word translated as valley
describes a streambed, frequently dry but could become (in the event of a downpour) a raging torrent. We are to picture Nehemiah scrambling up a steep creek-bed on the side of the mountain on which the city of Jerusalem was built. Somewhere upstream from Nehemiah’s location, the fathers had set foundations upon which they had built (and periodically repaired) the wall of Jerusalem (1 Kings 9:15; 2 Chronicles 32:5). It would appear that this section of wall needed the most work.
15 Then I went up in the night by the valley and inspected the wall, and I turned back and entered by the Valley Gate, and so returned.