And when they 'carbon date' a circle, how do they do it?

They don't do the circle, its what they find either within the circle, or the burials that provide the evidence of dating; radio carbon dating is a technique for determining the age of of carbon bearing materials (radioactive decay) which include wood and plants. So most reports will include rc dating with a plus and minus figure of a couple of hundred years. Dendochronolgy is tree ring dating, but also has problems with exact dating due to climate change having an effect on the trees.. Pottery is also used as a dating method, but its correlated with other sites.. Therefore its what you find in the construction of the circle that determines its date, and its affinity with other sites in the area.
Should get B+ for that :)