The Palace of Eternity was a large futuristic edifice, which existed in a dimension that operated outside the bounds of the space/time continuum.
The chronal conqueror known as the Lord of Time designed the Palace and used it as his base of operations. The inner workings of his lair were controlled by a highly advanced artificial intelligence known as the Eternity Brain. The Eternity Brain possessed the ability to open dimensional portals into any time period of its choosing, drawing resources which it would then use to defend the Palace. For example, should the palace come under attack, the Eternity Brain might summon a Tyrannosaur from the Mesozoic era to protect its battlements.
When the Eternity Brain became too much for the Lord of Time to handle, he attempted to shut it down. Failing that, he collected five warriors from various points in Earth's history to band together and disable the Eternity Brain for him. Informally known as the Five Warriors From Forever, these time-lost adventurers raided the palace, but were unable to stop the Eternity Brain's machinations. The Brain turned the Palace's defenses against them, capturing all five.
Shortly thereafter, members of both the Justice League of America and the Justice Society of America arrived at the Palace via the Cosmic Treadmill. The Eternity Brain activated the palace's laser defense grid, keeping most of the heroes at bay. Ultimately, the Elongated Man was the only hero capable of successfully threading his way through the palace into the central nerve cluster to deactivate the brain.
Although the Lord of Time was forced to abandon his palace following the destruction of the Eternity Brain, he eventually returned to it, in order to plot out future criminal exploits.