Gates of Hel

"“Aren’t the Gates to Hel closed?” "

"—Braunner"

The Gates of (or "to") Hel referred to a hypothetical link between the Grauwelt and Hel. It was believed by some to be an actual, physical structure.

History
Before the death of the Great Hund, the Gates of Hel were open for beings to pass both into and out of the underworld. With his dying breath, the Great Hund sealed the Gates of Hel forever, trapping his enemy, Dietrich, inside.

Passage into Hel
Only Mann was permitted to enter Hel when the Gates were still open. The Löwa, lacking an eternal spirit, were never allowed (or were never capable of) entry there.

Hel could only be reached either through death, or during life via the World Walk.