Navel of the World

The Navel of the World (世界のへそ Sekai no heso) is a location in Monster Girl Quest. Located in the very centre of the Sentora continent, it is

the entrance to Heaven.

Original World
The Navel of the World contains a large ivory tower that holds Heaven’s Gate at its centre, and a long path leading up to the tower. It is rarely visited by surface dwellers, but as a safety precaution to prevent people from discovering the gate, hundreds of Themis Traps guard the pathway.

Paradox World
In Monster Girl Quest Paradox, Sonya tells Ilias that the Navel of the World was consumed by the Tartarus rift during the Great Disaster 30 years ago, making Ilias questions if it is even still possible to access Heaven. The answer is yes, as the remainder of Heaven formed the newly discovered Snow Continent.

Trivia

 * The tower is never actually shown on-screen.