The light in the stairwell suddenly went out, forcing her to descend in the dark.

stairwell 是由 stair（楼梯）和 well（井）组成的复合名词，字面意思是"楼梯井"，指建筑物中环绕楼梯的垂直空间，通常包括楼梯本身和周围的墙壁。