Hades Gate

A Hades Gate is a giant doorway found in some scenarios of the campaigns of Age of Mythology and its expansions.

Overview
After defeating Kronos and the Titans, Zeus had them imprisoned in Tartarus, the deepest part of the Underworld. To ensure that they could not escape, all the paths into Tartarus were sealed by giant adamantine doors. Only a mortal capable of entering the Underworld can pry open the massive construct.