Wall to where?

This is a wall with a boarded up entrance to something.
The mysterious wall is about 20 meters away and at an angle to the support wall of the Korakuen (Marunouchi line) train station off the image to the left. It seems to be a retaining wall for the park above, but what’s inside the arch? There are stairs leading down the hillside from the park to the now-weedy area that was once a garden, judging from the plants running wild down there.
My best guess about this is that before Tokyo Dome was built in 1988, back when this area had a different stadium dating from 1952, this was part of that complex. Maybe it was the entrance to the Baseball Hall of Fame, or a pedestrian walkway from somewhere… but the Marunouchi line opened in 1959 and the train station would have been in the way then, too, woudn’t it?