The facility was set aside for the purpose of isolating and offering what treatment was available to a handful of beings diagnosed with an incorrigible criminal insanity. Governor Donald Cory oversaw 15 patients who met that description in the late 2260s, including Elba II's most notorious resident, former Starfleet Fleet Captain Garth of Izar.
With dangerous patients, Elba II had appropriately extensive security precautions. Patient living quarters were protected by individual force fields, and the colony dome itself was protected by a field that, when in place, prevented beaming and a could even withstand a phaser barrage from a Constitution-class starship in orbit. The toxic gases of Elba II's atmosphere provided a final daunting barrier.
None of the security measures precluded patients from attempting escape, as Garth tried in 2269. The mad, shapeshifting former officer ran circles around Gov. Cory and for a time, had the complete run of the facility, but never the means to get out. Even after the capture of Captain James T. Kirk and Commander Spock, with the Enterprise orbiting above, Garth and his followers never got closer to freedom than the steps they took from their cells.
A revolutionary new medicine, delivered by the Enterprise in her visit of 2269, promised to almost immediately reverse arterial and brain damage and was hoped to aid the rehabilitation of the wards of Elba II. (TOS: "Whom Gods Destroy")