The mountain pass road was narrow, and moreover, steeply graded.
Literal
Mountain-crossing [possessive-の] road [topic-は] narrow, moreover steep-gradient was.
しかも (moreover/what's more) adds a second negative quality on top of the first. 狭く is the negative conjunctive of 狭い, linking it to the next clause. 急勾配 (steep gradient) is a geographic/engineering term. 山越え (mountain crossing) is a compound noun.