You look at the table, and are about to turn away, when something catches your eye. There are words carved into the table-
I know the secret of the wall,
so hear my riddle and catch your fall.
Two triangles connected, often glass,
As I show the hour pass.
And in this riddle, I am shown,
Five in both, in both alone.
You decide that the answer to the riddle has something to do with the wall, and decide to figure out what, and go examine the wall.