The NONA Tower is owned by the Ministry of Health and Welfare. According to the official blueprints, it is 70 stories high and contains floors of offices as well as an immense radio tower on its roof. Unofficially, it is where the Sibyl System is housed.
The official blueprints provided by the government indicate that the tower's basement goes no further than four floors. In reality, the basement extends 20 stories down, providing ample space to house the Sibyl System. Access beyond -4F is denied physically by a series of moving wall sections that can seal off the lower floors. Access is only granted by the Sibyl System and involves more complicated judgments than simply one's Crime Coefficient or Psycho-Pass.
Once below -4F, the stairs extend haphazardly below. At the center of the unit is the core of the Sibyl System, which houses the minds of its criminally asymptomatic members. Its thick exterior shell and additional gates prevent trespassers from entering into the core accidentally or purposefully.
The NONA Tower maximizes Sibyl's power whilst remaining hidden from the public eye. The radio tower on the roof enables Sibyl to make judgements and assessments throughout Tokyo without hard-wiring, an outdated modality. With the NONA Tower being the highest structure in 22nd century Tokyo, there are no connection issues save for buildings that are purposely offline to reduce the risk of being hacked. Through the NONA Tower, Sibyl sees and judges all.
To date, only three individuals have been granted access by Sibyl beyond -4F and into the core. Inspector Akane Tsunemori is given permission on two occasions: the first, in order to strengthen her resolve to lead CID's Division 1 to capture Shogo Makishima and the second, to accompany Kirito Kamui. Kamui is granted access so Sibyl can judge him and allow itself to be judged in order to solve the omnipotence paradox and evolve. MWPSB's Chief Joshu Kasei is granted access as she is the bodily representation of the System's members. The rest who enter beyond -4F have either hijacked their way through or followed the doors previous hacked. Gu-sung Choe, under Makishima's orders, hacked his way through Sibyl's physical defences to make his way to the core. There he took a video he intended to release to the general public, though he's killed shortly after filming. Enforcer Shusei Kagari enters by following Choe's steps. Likewise, Enforcer Sakuya Togane follows in Tsunemori and Kamui's footsteps.
Though the signal from the NONA Tower reaches Tokyo and the surrounding regions, it does not extend to the floors below -4F without Sibyl's permission. Initially, Kagari's Dominator fails to work due to the apparent lack of connection, but is later used to execute him. The Dominators wielded by Kamui and Tsunemori are also in working order. It may be speculated that Kamui's Dominator is needed to judge Sibyl whereas Tsunemori's Dominator is a communications device between Sibyl and herself. Both Kamui and Togane's Dominators are granted connection so that they're able to execute one another. Still, the control by Sibyl over its connection below -4F remains a theory.