Yggdrasil

Yggdrasil (ユグドラシル) is the sanctum where the original X sealed the Dark Elf. Zero fights against Elpizo in this locale in the end of Mega Man Zero 2.

Characteristics
Concept art depicts Yggdrassil as a gigantic tower built around an equally large tree-like structure, which seems to symbolize the world tree from Norse mythology. Located in the very center of Neo Arcadia, Yggdrassil is protected to the point that access is denied to even the high executives, and for good reason: at the top lies the chamber where the body of the original X acts as the seal of the Dark Elf.

The interior of Yggdrassil has a relatively dark background, with cross shaped columns protuding from the ceiling. The tower has several levels separated by rooms with teleporters, which lead the player to rematches against the Mutos Reploids who are now under Elpizo's influence. The floor and walls of the first level are dark gray, with violet plates connected by what appears to be power lines which pulse with teal blue energy. At higher levels, the colors change to light gray and purple, eventually becoming white and navy blue.

The Dark Elf's chamber is a large room with a massive silver structure at the center. The Dark Elf is contained in a golden sphere with plating creating an "X" pattern on it, positioned above a blue capsule storing X's body. The structure is surrounded by a series of pixelated rings of light colored green, representing the tree's leaves.

Background
After the events of the Elf Wars, there was a dire need to contain the Dark Elf, the Cyber-elf that nearly led to the world's destruction. Years prior to the events of the Mega Man Zero series, the original X eventually saw the need to use his own body to maintain the seal in Yggdrassil; although the effort worked, the shock split X's soul into five Cyber-elves; four of these would be used in the creation of the Four Guardians.

Mega Man Zero 2
After the events of Operation Righteous Strike, Elpizo is seduced by the power of the Dark Elf and abandons the Resistance, making use of the Baby Elves Crea and Prea to methodically break down Neo Arcadia's defenses in order to reach the Dark Elf. He eventually manages to make his way through the Four Guardians and infiltrate Yggdrassil. Cyber Elf X makes an attempt to stop Elpizo, but he is repelled by Crea and Prea. Elpizo then waits for Zero to arrive, only to immobilize him and destroy X's body in front of his eyes. He then takes the released Dark Elf within his body, using her power to transform and fight Zero. After a violent battle, Elpizo is defeated by Zero.

With his body beyond repair, Elpizo apologies to Zero for what he had done, which somehow compels the Dark Elf to save his life by turning him into a Cyber-elf before flying away.

Layout
As a tower, Yggdrassil has a vertical flow with ladders as the main route to the upper levels. It can be divided in three sections, which are split by teleporter rooms leading to the rematches against the Mutos Reploids of the game.

Section 1
This part is fairly straightforward, with little to no hazards. Glare Eyes and Pantheon Hoppers are the main obstacles. This section leads to a room with three teleporters which take the player to the following bosses: They must be all defeated to unlock the next section.
 * Top: Hyleg Ourobockle
 * Left: Poler Kamrous
 * Right: Phoenix Magnion

Section 2
This area does not differ too much from the previous one. Panel Cannons and Pantheon Hunters are the main enemies, and a few spike beds provide additional hindrances. This section ends with another teleporter room leading to the following rematches: Once again, defeating them is mandatory in order to advance.
 * Top: Panter Flauclaws
 * Left: Kuwagust Anchus & Herculious Anchortus
 * Right: Burble Hekelot

Section 3
Spikes are much more prominent on the floor and walls of this section. Securipiders lurk on the walls and ceilling, while Pantheon Guardians patrol the floors. This section leads to the Dark Elf's chamber, where Elpizo awaits.

Notes and Trivia

 * The name "Yggdrassil" is officially spelled as such in the Mega Man Zero Official Complete Works, even though the mythological tree is named "Yggdrasil" (with one "s" instead of two). In RTRZ - Idea, the name is spelled "Yggr-drasill".
 * Yggdrassil has not been seen or mentioned after the events of Mega Man Zero 2, but it is presumed to have been obliterated by Ragnarok along with Neo Arcadia.
 * Yggdrassil's theme (Silver Wolf) was remixed for RTRZ - Idea and RTRZ - Mythos.
 * The Tower of Verdure of Mega Man ZX Advent is another building with a giant tree inside, although in this case, the tower is riddled with overgrown vegetation. It should not be confused with Yggdrassil.
 * Yggdrassil is one of the few stages in Mega Man games where two individual bosses are fought simultaneously.