The Temen-ni-gru is a big tower built by the diciples of the Demon Clan, used as a path from the Human World to the Demon World. It was selaled away by the legendary Dark Knight Sparda, with the use of a sacred and difficult ritual that required, among other things, to sacrifice his own power and blood and the same things from a priestess named Mary.
The Temen-ni-gru has several floors underground and above ground, making it a large place to go through. Despite being built by demons, it has several things that humans uses, such as prisions, normal rooms, dinning rooms, and even elevators.
There are several statues all around the place, which represents the seven capital sins.
The person that manages to break all the seals that Sparda put on this tower, will get the powers of the legendary Demon and his complete form.