From Lexicanum

The Eternity Gate is the Imperial Palace's final gateway to the Sanctum Imperialis and its Golden Throne, and the Emperor himself. Two Imperial Titans of the honored Legio Ignatum stand guard, an eternal vigil they share with the 10,000 guards of the Adeptus Custodes.

Along the grand hallway to the Gate, thousands upon thousands of banners line the way, those of the Emperor's servants: The Space Marines and the Imperial Guard. Just as they honour the Emperor, he returns such honour.

At the end of this hallway of heroes the gate itself stands, massive and unbreakable, forged of tempered adamantium layered with ceramite. Its outer face is sheathed in gold, and masked with an image of the Emperor in the days of the Great Crusade standing triumphant over his broken enemies.

An elite core of 300 Adeptus Custodes called the Companions stand guard over the Emperor himself while the rest of the Adeptus Custodes guard the rest of the Imperial Palace.

During the Battle of Terra in the Horus Heresy, Primarch Sanguinius alone defended this entrance from the hordes of Chaos vying to overrun the Palace when no other could withstand the ferocity of the assault.

Sources

• Inquisition War (Novel Series), Inquisitor (Novel), by Ian Watson
• Warhammer 40,000 3rd Edition Rulebook
• Warhammer 40,000 4th Edition Rulebook