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Magical circles are meant to keep things inside, & other things outside of them.

As are all other walls.  The walls of your home are meant to keep out the elements, enemies, etc & keep in family, warmth, love, etc.  The Berlin Wall was meant to keep the West, non communist peoples out, the Russian controlled people in, Hadrian’s Wall to keep the Picts out, that one not so much to keep other people in.  A magical circle is not built with wood, rocks, etc., but it works in much the same way.  A magical circle is built with ones will, using the 4 elements, according to ones chosen tradition (some traditions go from the North first, others from the East) & always done deosil unless it is set up for black magic, (not recommended) or for certain banishing rituals.  Circles are set up to keep inside of them the power that is raised during the ritual until it is ready to be released, & they are set up to keep the solitary witch or coven members safe from all those mischievous spirits that are drawn to power, some of which are more hurtful than others.  While setting up the circle any spirits who are wanted inside of it are invited to participate.  Like all walls circles must come down eventually, & that is done with care & according to the tradition it is being performed within.  A sudden downing of a circle can have a backlash upon the one(s) who put it up, so unless it is an emergency it should be taken down with as much time & effort as went into putting it up.