A magic circle is an aid to casting. It contains a kind of shorthand for the mental processes a caster must go through in order to properly tune their mind to the music of the spheres (that is, to the magical field generated by the planet).
One of the first things a fledgling sorcerer or priest learns is how to interpret these complex symbols. They’re often composed of more than one circle, interlocking or concentric, and each spell has a unique circle and/or combination of Words, if the caster is a sorcerer. By memorizing the necessary gymnastics of mind, the caster memorizes the spell; otherwise, they need the circle visible.
Magic circles may be inscribed on paper, vellum, wood, stone, or metal. In the case of enchanted items, such as message domes or magic swords, the circle often takes the form of a fine wire of precious metal inlay in the item.
Magic on an epic scale always requires a circle scribed in rock, preferably bedrock or some other, single stone. See the prologue to Saga of Menyoral: Hard Luck for more. You can have a look here: Excerpt from Hard Luck.