Hell, or Niflheim, is the realm of the demons and where Hild rules but currently it is being ruled by Hagall who has overthrown Hild with a rebellion. While showing to have surreal architecture, it isn't as dark as other conservative interprations of Hell. So far there hasn't been any souls, human or otherwise, in active torment. Which may mean that this series doesn't present Hell as an afterlife, but simply as another realm.
There are various structures around including a large spire which is presumably the seat of the Daimakaichō's' power. There are also plantlike structures that can serve as prisons to individuals.