Yggdrasil is a planet existing in the Rootverse, and the home of a race of aliens who created the humans. Yggdrasil is the home planet of this race, but they make use of teleporter technology to link Yggdrasil to the seven other planets in the solar system, and also to Earth.
The most defining feature of Yggdrasil is the large tree growing from it, that is large enough to be visible from space. The roots of this tree travel throughout the entirety of the planet, and inside these roots are homes which aliens live in. The core of the planet functions as the core of Earth, but there are no tectonic plates on the planet (or if there were, the tree's roots merged them all together).
At the top of the tree is a large city (known to the Norse as "Asgard") built by the aliens, which is where those who are higher up in society live. Their king lives in a castle, called Valhalla.