Background Information

The arcs:

Assiah (Volume 1 - Volume 4)
Hades (Volume 4 - Volume 7)
Jahhanam (Volume 7 - Volume 11)
Yetzirah (Volume 11 - Volume 14)
Briah (Volume 14 - Volume 16)
Atziluth (Volume 16 - Volume 20)

There are 9 orders of Angels:

Seraphim
Cherubim
Thrones
Dominions
Virtues
Powers
Principalities
Archangels
Angel

There are 4 element Angels:

Earth- Uriel
Water- Gabriel
Fire- Michael
Wind- Raphael

Yggdrasil is also known as Uchyju, it means the "Tree of the Universe"
Anagura is the name of the first level of Hell below earth.
Gehenna is the name of Kurai's kingdom.
Anima Mundi is the "Soul of the World".

I found this map (which includes the Heavens, the Hells and the Assiah) somewhere on the net. "A picture is worth a thousand words" SO, VERY, TRUE! Take a look at the map, it might answer some of the questions you had in mind. ^^