Found in Shinjuku National Park, it is under guard by both the Ashura-kai and demons to keep the three prisoners there from being rescued. The three prisoners wear masks and are kept in cages, which the demons refer to as birdcages. A barrier is placed in the park to keep people from getting close, and an Ashura-kai member guards the place with his Balor. The first masked man is under the guard of Murmur, the second under Gemori, and the third under Asmodeus.
According to a Hunter the tower appeared one day after a fight between several demons happened in the park. Rumors also spread of three angels who attacked Tokyo but were supposedly weakened and defeated instead. Considering the timing of King Ahazuya Mikado's dethronement and that the mask can only be removed by the prisoners' kindred spirits, all suggest that the three prisoners rescued by Flynn from the tower under Gabby's request are in fact Uriel, Raphael and Michael.
This is eventually confirmed in the Clipped Wings DLC; at the end of the second mission into Mikado's past, Mastema, shedding his guise as M, ushers the sealed Archangels into the tower's birdcages after fitting them with the masks they are eventually seen wearing when releasing them.