Ironlock Prison is a massive abandoned prison complex situated in the Dark Swamp. According to Nicole, it was constructed in 1543 and abandoned "hundred years ago". (According to the timeline given in "Blast to the Past, Part 1", this suggests the prison was abandoned in 3134.)
Ironlock Prion's s main entrance lies across a deep gorge, spanned by a rotten wood drawbridge. It resembles a foreboding castle, and even Sonic claimed that it looked creepy. During the events of the series, Ironlock is in serious disrepair, with crumbling walls and ruined buildings. Where it remains intact, it can be seen that there were various types of prison cell. Some doors and cells are constructed of metal bars, while others are constructed of stone block walls, and have reinforced wooden doors.
During a mission to Robotropolis, Sally received a message she believed to be from her father, King Acorn. Nicole claimed that the source of the signal was in the Dark Swamp, so Sally and Antoine went there to investigate. Learning that Ironlock was the only known structure within the Dark Swamp, Sally directed the investigation there, despite Antoine's repeated suggestions that they should just go home to Knothole. When Sally and Antoine crossed the drawbridge, the wood finally gave out. Although they made it to the other side, the bridge deteriorated and fell into the chasm. When Sonic arrived later, he was forced to leap over the gorge to gain entry.
During their searching, Sally found a scrap of cloth from her father's cloak, as well as an ancient computer that she believed had been used to send her the signal. A longer, but still incomplete, message was recovered from the computer, along with schematics for all of the subterranean tunnels throughout Robotropolis. However, a loose stone landed on the computer before the three Freedom Fighters could leave, opening a trapdoor that plunged them into the sewer system far below.
The tunnel system underneath Ironlock was guarded by a robot disguised as a one-eyed, reptilian monster. Sonic was able to defeat the robot by using a Power Ring. The walls and ceilings in some of the tunnels connected to this area are metal, and those tunnels are lit with soft red lights. Using the maps obtained from the Ironlock computer, Sally was able to navigate the tunnels all the way back to Robotropolis.