City of Light was created to a location where a meteor fell, during the last planetary alignment, some 3600 years ago. From that meteor they also forged the Triangle of Light. The People of the Light worshiped the Triangle of Light, for its power to control time. Abuse of that power lead to the destruction of the City of Light. The few survivors split the Triangle and hid in the opposite ends of the world, to ensure it is never used again.
One of the halves was left inside the mountain range now called "dead zone". The location was revealed to be in today's Siberia. When Lara Croft gathered the Triangle of Light halves, she and the Illuminati members traveled to the island that once housed the City of Light and reforged the Triangle of Light. Lara Croft however later destroyed the Triangle, so no one could abuse it's power.