Archaeological evidence shows there were settlements in the vicinity of Kandor going all the way back to Krypton's prehistory. The Crystal Hills region contains caves that contained demonic cult activity. It is known that these people (primitive by modern Kryptonian standards) had electronic technology, including holography.
Eventually Kandor grew to become one of the largest cities on Krypton.
Shrinking of Kandor
In the late 1950s, an intergalactic criminal called Brainiac used a shrinking ray on Kandor (and the cities of many other worlds) to miniaturize it and store it in a bottle for space travel. Brainiac intended to restore the cities on a composite world which he would rule over - a project that would take many years to complete.
Sometime after the shrinking of Kandor, and after Krypton itself was destroyed, Superman encountered Brainiac and took the Bottle City of Kandor back to Earth for safekeeping in his Fortress of Solitude, until such time as he can restore the city to full size.
Years later, when a kryptonite warhead decimated Superman's secret sanctuary, desperate Kandorian scientists had no choice but to cast their bottle city adrift in the Phantom Zone.
Superman has since collected the city and placed it in a bottle containment once more. In All Star Superman #10 Superman let the hero Quintum go inside of Kandor and speak to the Council. They then decide to let Quintum take Kandor to Mars so they can live there as super-people, away from human society.
Superman explored the remains of Kandor while visiting an alternate Krypton before its destruction. Instead of finding the ruins of the once great capital razed by Black Zero's nuclear device, the Man of Steel also learned that Kandor - prior to its trans-dimensional disappearance - was a vast internment camp for aliens.
The mighty Ceritak, son of gentle Cerimul, a late Elder of Kandor's ruling Council. While Cerimul struggled to unite Kandor in peace following its liberation from Tolos, Ceritak escaped the bottle city and fled to Metropolis. His misunderstood attempts to overcome the language barrier earned the horned Kandorian the nickname "Scorn" from the Metropolitan media chronicling his exploits.
Points of Interest
The Crystal Hills of Kandor - The Crystal Hills is a restricted historic zone. The old historic scrolls say the hills were a sacred area in prehistoric times. Remains left by the Torgs, a Kryptonian prehistoric cult that worshiped the demon-god Kvorg, including ancient writings and luminescent statuary can be found in caves in the area.
Arvor - a top inventor
Ken-Dal - a renowned scientist
Yllura - an accomplished archaeologist