Ed,
Although I would hate to question the veracity of a Wikipedia article, let me give my more plebian sources a whirl.
Generally speaking, throughout the ages (magical circle casting actually predates the Classical era) there are two different types of magic circles cast. One is a ceremonial circle designed to keep the spellcaster protected from whatever they conjure up, and the other is a spiritual space designed so that the spellcaster can commune with his/her god/goddess. The first type is what you're asking about, while the seond is what is primarily used by practicing pagans today.
Now then; theoretically, the protective magical circle is actually an invisible sphere, built arounf the spellcaster in the center. It is a magical environment created and built fromthe spellcaster's personal power. If you imagine a sphere of evenly spaced rays of light, for example, the person would be inside of it like a gerbil in his cat-proof walking ball (sorry Ferret--had to use a different rodent as an example
). If you look at it THAT way, then a spellcaster who is powerful enough could concievably create a magical circle that could repel physical forms of attack as well as supernatural ones.
Aleister Crowley (restrains the rolling of her eyes) babbled on a bit about spell circles and I don't remember a darn thing about what he said specifically. However, Heinrich Cornelius Agrippa )1486-1585) in his De Occultia Philosophia -p 1583 said:
A circle doth answer to unity, and the number ten; for unity is the center, and circumference of all things; and the number ten being heaped together returns into a unity from whence it had its beginning, being the end, and complement of all numbers. A circle is called an infinite line in which there is no terminus a quo nor terminus ad quem, whose beginning and end is in every point, whence also a circular motion is called infinite, not according to time but according to place; hence a circular being the largest and perfectest of all is judged to be most fit for bindings and conjurations; whence they who adjure evil spirits are wont to environ themselves about with a circle.
Agrippa is a real kick, by the way. The loony old bird was a lot of help to me when I was researching something 'odd' a while back.
To make a long story short, then, if you look at the magic circle as more of a magic sphere it might help you twist your thoughts around a conjured power repelling a physical attack. I have to admit, though: I think the image of a demon chucking rocks at a sorcerer protected by his magic circle is hysterical. it almost has a Goldman/Princess Bride sort of edge to it. *grin* I found a lot of great stuff on
www.esotericarchives.com/esoteric.htm if you want to go digging about some more. It's one of my favorite source sites for magic, folklore, witchcraft, and so forth. If you actually have the time and patience to go picking through the site, you can get resonably useful translations of some old medieval supernatural texts.