Circles have been sources of power for centuries. Old temples are full of circular patterns and gatherings have always taken place in circles. Mysticism surrounded their perfect symmetry (ie no matter how you fold a circle through its centre, the two halves will always be perfectly equal). It was considered polite in old times to seat your guests at a circular table as it did not require someone to act as the head of the table. Cicles in magick are often called circles, domes, or cones of power. You can visualise it as a dome or cone.
Creating a circle around the area in which you wish to work creates a sacred and protected space: it both stops your energy seeping out, and stops other energies seeping in.
There are many who don't believe that circle casting is necessary, others believe that it is vital. It really depends on the type of magick you perform. For long and detailed rituals, such as the sabbats or initiations, a circle can be a good idea. You can modify a circle casting to create a banishing ritual in itself (by expanding the circle to fill a negatively charged area). However, if you are like me and use spells all the time during the day to get the traffic rolling and help you with other little things, you'll know that a full casting is simply not practical. You may choose to combine the practices. You can save casting for certain spells that you think it will help with, and leave them for those that don't need it.
Circles casting may also be modified to create general protection domes if you are feeling unsafe. They fade with time, so you don't have to worry about breaking them. A few years ago I was having a really bad time sleeping. The energies in my room were disturbing me too much and I'd lie there wondering what was about to leap out at me. By casting a quick circle of light around myself and my bed I found it much easier to sleep. They usually faded by morning. (In time you will be able to tell when they are still there.)
Circles can also help you focus your intent in order to get a maximum amount of energy, while preventing the area around you from becoming tainted by passing spirits and energies.
There are many, many versions of casting a circle. You can cast circles with specific intentions, or for different times (such as nocturnal circles). Other versions also call the Guardians of the Watchtowers (the four quarters) to assist.
You should also note that you don't have to cast a new circle for every spell you do. You can do as much as you need to in the same circle and just close it when you're done with magick. (Oh, that doesn't mean you should leave it permanently. Just take it down once you've finished working, the same as you might turn off your desk lamp once you've done with your paper work).
I have placed the rituals in the ritual book, from casting, leaving and entering and closing the circle.
