Riddler

Context: In the 12-part storyline "Hush", it is revealed that Riddler suffers from cancer, which also afflicted the mother of Dr. Thomas Elliot. Riddler uses one of Ra's al Ghul's Lazarus Pits to rid himself of the disease, and offers Elliot the chance to cure his mother as well, provided he pays a large sum of money. However, Elliott is in fact eager for his mother to die in order to inherit her fortune. Elliott, who goes on to secretly become the masked criminal Hush, explains he wants to get revenge on his childhood friend Bruce Wayne. The two of them agree to work together and Riddler sets Killer Croc, Poison Ivy, Harley Quinn, Joker, Clayface and Scarecrow out to destroy Batman, with Ra's and Talia al Ghul and Lady Shiva being temporarily drawn into the scheme as well.  During the psychotic break that follows exposure to the Lazarus Pit, the Riddler deduces Batman's secret identity and that the late Jason Todd was once Robin; he then reveals his knowledge to Hush. He has Clayface shapeshift into the form of Todd in order to torment Batman haunted by the former Robin's death. Batman first thinks that Riddler had stolen Todd's corpse and hid it outside of Gotham Cemetery, but it was revealed in a later storyline, Batman: Under the Hood, that Todd was alive the whole time and had personally played a role in Riddler and Hush's scheme.  When the Riddler threatens to expose Batman's secret identity, the Caped Crusader mockingly labels it an empty threat, pointing out that if Riddler revealed the answer to the riddle "who is Batman?", it would become worthless, something Riddler would not be able to stand. In addition, Batman warns him that if he reveals the secret, it would give Ra's al Ghul a vital clue that he used a Lazarus Pit without his permission, and the League of Assassins would subsequently retaliate against him.  In Tony Daniel's "Life After Death", Riddler appears early in the story at a gala party attended by Arkham, Dick Grayson, Huntress, and Oracle, hired by Penguin to find the Black Mask. As he chases Catgirl away from her robbery of the venue, the Riddler is rocked by a bomb detonation, the trauma re-awakening his psychosis. Cackling, rambling and insane, he terminates his contract with Cobblepot and disappears.  In "Riddle Me This", the Riddler still "acts" as private eye and teams with Batman to solve the murders of a mysterious sorcerer named Sebastian Rothschild (aka Sebastian Blackspell). Blackspell is apprehended, but only after Batman suspects Riddler went to great lengths to orchestrate the ordeal, including poisoning himself with a nearly lethal dose of Joker gas to skirt suspicion and act on a grudge between him and Blackspell.  Riddler's return to villainy is cemented in "Eye of the Beholder". Investigating the Sensei's attack on the Jade Society, Batman (Dick Grayson) is ambushed by Riddler and a young woman introduced as Enigma, Riddler's daughter. Riddler and Enigma escape, delivering the Jade Society member list to Gilda Dent. Riddler is paid, but is more interested in another reward, which Gilda promises after one more job. This occurs in "Pieces", where Gilda reveals herself to her estranged husband Harvey, who is now the disfigured criminal Two-Face. She hires Riddler and Enigma to help Two-Face best Mario Falcone and reclaim his coin. The plan works; Riddler and Enigma defeat Batman and reunite the Dents. The Riddler is rewarded with multiple dossiers of himself. When Enigma calls him a has-been, Riddler retorts with a new riddle: "What's green and purple and bleeds profusely?". Enigma's response is cut short by her scream, implying that the Riddler has murdered his own daughter.  In DC's 2011 relaunch of all of its monthly titles, The New 52, Riddler appears as an inmate at Arkham Asylum in Batman #1. Redesigned in the style of the new titles, he sports a green mohawk in the shape of a question mark.  Riddler appears in more traditional form in the short that concludes Batman #15 "And Here's the Kicker", the third part of "Death of the Family". After it is revealed that Joker has secretly hijacked Arkham Asylum, Riddler is depicted as a current inmate, calmly biding his time and taunting guards. But when Joker appears and reveals his great respect for Riddler (as the villain whose dangerous intellect has kept Batman "sharp"), he uses Joker-gas to force Riddler to prove he could have escaped his cell anytime he wanted. To his chagrin, Riddler does, and becomes quite alarmed when Joker shares a write-up of his plan to ambush Batman. Joker admits Riddler will have little part in his designs, but should stick around for the "show" anyway.  The Riddler made an appearance in Batman, vol. 2, #21, the opening book of the "Zero Year" arc, where his surname is changed from Edward Nigma or Nashton to Nygma. The Riddler later appears in both the second and third chapters of the "Zero Year" storyline. In the canon, the Riddler is Batman's first masked supervillain, and is not only able to best Batman twice, but also takes control of Gotham, causing it to become a flooded wasteland where only the intelligent are meant to survive. Although the Riddler continues to be steps ahead of the Dark Knight, he is eventually defeated by the combined efforts of Batman, Commissioner James Gordon and Wayne Enterprises SEO Lucius Fox. He is later moved to Arkham Asylum.  Riddler appeared one more time in New 52 in the last three issues of Flash. Holding a presumed dead Heat Wave hostage, it is revealed that the Riddler designed a range of deadly drones around Central City, drones that he had out-sourced to the CCPD. Alongside the Trickster (whose arm Edward had placed a bomb in), Riddler begins ruthlessly punching and beating the Flash (Barry Allen) before the speedster is quickly rescued by the Pied Piper. Riddler then threatens to have his drones open fire on the citizens of Central City once more. However he is eventually defeated and imprisoned by a uniformed effort between the Flash and the Rogues. CANNOTANSWER

Question: What were the drones that the Riddler designed around Central City for?
Answer: Riddler then threatens to have his drones open fire on the citizens of Central City once more.