Magic Runes

Magic Runes was a form of writing written in a bizarre, runic script that was capable of casting magic spells without the deliberate activation of them. Princess Eris had a circle of runes in her castle to catch trespassers.

Linguistics
It's unknown whether or not the runic language translated into any form of spoken language or if it was purely for magic purposes. The runes themselves appeared to be similar to the modern Equestrian script with noticable differences. Familiar glyphs such as runes resembling "3", "S" and "N" can be easily seen, but several others appear to be completely different.

The script seemed to have a logographic element to it as well, with high emphasis on the Sun icon and shapes like a triangle and circle, which were not apart of the proper script.

Certain combination of runes can cause magic spells to occur, such as a trapping spell.

Behind the scenes
The magic circle appears to be based around the basic design of Runic scrips and the idea of a "magic language", a popular idea in occultism. In general, the symbol seems to be based off occult ideas, such as a design obviously based off a pentagram and the idea of casting magic through written words.