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.
But the project never became a reality: a rogue scientific trained by the Lanteans, named Dorane, posing as a Lantean, manipulated the local population to build a bio-weapon experiment lab under the complex, using the natives as test subjects, and turning the people named Koan into a highly mutated and savage creatures. The Lanteans, after uncovering what happened, attacked the planet and destroyed most of the complex and the laboratory, evacuating the surviving population and leaving Dorane and the Koan imprisoned in the planet.
10,000 years later, 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. Sheppard's team fight the Koan and finally killed Dorane, liberating the Koan from its control. (SGA: "Reliquary")