When Vitam was first colonized by humans in the Age of Strife, Nidum was just an unnamed research facility. It would have been built several hundred metres underground to avoid interfering with the planets extra-ordinary wild life. As time went on, the facility slowly grew and bacame the main base of operation on Nidum, with a medical centre, small barracks and extensive laboratories and animal pens. The entire facility remained underground.
Unfortunately when Vitam lost contact with the rest of humanity the few pcokets of human life slowly became more and more tribal. Over generations, the technology failed them and they started living in tribal clans above ground.
Thousands of years later when the planet was rediscovered during the great crusade, Nidum was refitted and once again used as a central point to the planet. The other small stations and underground facilities across the planet were slowly re-fitted. Nidum saw the most expansion. Within 100 years the underground facilty had doubled in size.
Nidum was named the capital of the planet. The city was still mostly home to scientists and imperial researchers.
Unfortunaterly Nidum's growth would be stifled again when all it's funding was cut to support the war effort during the Horus Heresy. As small group of faciliteis remained functional across the planet. Much of Nidum was closed of as there were not enough pople to run the city.
In the beginning of M36, Nidum found themselves host to a fleeing Ork warboss. The ork was closely followed by a strike force of Space Marines. Once the Space Marines dealt with the Ork they left again. The Commander of the Space Marine force was soon asked to lead hos own successor chapter. He chose Vitam as the Chapter's home planet and nominated Nidum as the location for his fortress Monastery.
The city saw it greatest expansion during this time. All its facilities and defenses were increased and the underground city expanded in leaps and bounds.
The City is currently the only settlement on the planet that is home to more thaqn 80 people. It's population is almost 6000. The city is equipped with labs, a hospital, underground farms, a barracks and housing. The mighty fortress monastery also resides at the centre of the settlement.
Nidum is the only settlement on the planet larger than a village, and is built almost entirely underground. This design was chosen so that humans could live on Vitam without disrupting the nature and wildlife.