Sentient life was plentiful on the planet, but on a most primitive level. According to probes, the planet had no signs of industrial development or organized civilization. As the probes detected primitive humanoids at irregular intervals, the instrumentation of these spacecraft also detected regular pulsations of generated energy.
Overall, however, Sigma Draconis VI was a remarkable example of a retrograde civilization. At the peak, it was advanced beyond 23rd century Federation capabilities, but had de-evolved to primitive level, beginning 10,000 years ago when a glacial age reoccurred. An underground complex was developed for the women, while the men remained above, causing a male-female schism. This ultimately caused a cultural development which was last seen on old Earth when the Romans were warring with barbarians.
The foundations for a dead and buried city were later found near a elevator built into a cave. Several hundred meters below the surface, they discovered a highly complex civilization built around the high energy generation source, based on ion power.
This civilization was discovered to be the home to what the Morgs referred to as the "givers of pain and delight," otherwise known as the Eymorgs.
The civilization, itself, was controlled by the Controller, which was responsible for keeping the Eymorgs alive for a period of 10,000 years, and for whom the Eymorgs lived to serve.
Following the disconnection of the Controller (which was "powered" by Spock's brain), by the crew of the Enterprise, the Morgs and Eymorgs were force to live together once again, in a forced attempt to re-establish a civilization. (TOS: "Spock's Brain")
For the 2007 remastered version of the episode "Spock's Brain", an all-new computer-generated version of Sigma Draconis VI was rendered. The new, more realistic planet featured landscape more consistent with the look of the surface. Specifically, its non-ice-covered portions were changed from a blue hue that suggested large oceans to an ocher that matched the color of the surface. The glacier-free areas were also more narrowly concentrated around the tropics of the planet, as the script suggested they should have been.
In addition to new shots of the Enterprise entering and leaving orbit of the planet, old footage of the landing party beaming to the surface was tracked into a brand new matte painting which expanded the scope of the original shot, showing a more mountainous and snow-covered landscape. 
While in command and recording his only log entry in TOS, Sulu inadvertently called the planet "Sigma Draconis VII". Kirk also inadvertently refers to the planet as "Sigma Draconis VII" in his own later captain's log, even though he himself referred to the planet as "VI" twice earlier in the episode.
According to Star Trek: Star Charts, Sigma Draconis VI was located in Sector 004 (Sigma Draconis sector). The Morg and the Eymorg formed a congress in 2268 after the crew of the USS Enterprise reintegrated the civilization. In 2370, there were an estimated 1.3 million Morg and Eymorg living on Sigma Draconis VI.