It was the deep air raid shelter for the three 6" gun emplacement at Fan Hole. Completed in Aug. 1941 for the four officers and 185 men of the 203rd Coast Battery RA who manned the site.
Here's a few of my photos.
View from the bottom step:
Looking back up:
To the right:
Light
And switch:
One of the unlined sections:
Tanks.
WW2 Aerial shot.