Metatron's Cube
The sigil which opens the portal to heaven is a design which is known as Metaron's Cube, although it has not explicitly been referred to as such.
After Metatron expelled all the angels and locked the Gates of Heaven, he created a piece of spell work that allowed him to come and go from Heaven to Earth [1]. The spell simply requires a specific sigil to be drawn. Once done, it will create a doorway into Heaven.
The playground where the portal is located in a playground. It is always guarded by one or two angels,and they draw the Metatron's Cube sigil in a sandbox.
As Metatron's spell has not be reversed, even after his capture, this is the only method for which angels can access Heaven from Earth.
Metatron's Cube in Lore
Metaron's Cube is a a geometric figure composed of 13 equal circles with lines from the center of each circle extending out to the centers of the other 12 circles. It is an example of sacred geometry.
