Heliopolis II is a castle, similar to that of Heliopolis in the Milky Way, on a planet in the Pegasus galaxy, which was nicknamed Heliopolis II. The Atlantis expedition discovered that this planet had also been used as a meeting place, but rather than between the Asgard, Nox, and Furlings, it had been between races of the Pegasus galaxy.
While exploring the unused sections of Atlantis, Major John Sheppard and Dr. Rodney McKay stumbled on a recording device that revealed the Stargate address to this planet. Believing that the address may lead them to a vast repository of Ancient knowledge, the team embarked on a mission to this uncharted world. There, they discovered this ruined city, full of whispered secrets and dark shadows. (SGA: "Reliquary")