The six sephiroth of Yetzirah with the Sun at center forms a perfect hexagram directly in the middle of the tree. The legion of Seraphim corresponds to the fifth emanation and the top left point of the hexagram.
The Seraphim are sometimes referred as the guardians of Gods' presence which may also be interpreted as saying that they are guardians of the abyss, and yields a familiar HGA acronym. Although in this particular context representing Holy Guardians (of the) Abyss...
The Wall