When you cast a circle, you are in fact no longer in this world entirely. You are also not in the astral world entirely. You actually are in between the two.
When you are inside the circle you are in a sacred space. If you have ever been in one, you know this. Some people are able to see the walls of the circle and the world beyond appears misty or etherial. Some simply sense the change in the space as the circle is formed without seeing any difference. If you havent had this experience yet, you will. As you develop your abilities to visualise, concentrate and open your senses you will more than likely come to a place where the difference will be apparent to you. Even if it is not, you must know that there is a difference. The circle is real, the walls are there, concentrating the power within them until is is released as a fully formed energy form to the universe.
The circle is a powerful thing if you make it one. Casting a circle is a skill that you develop with practice. Your abilities will increase with time and the circles you cast will be stronger and more powerful than the first ones you began casting as a beginner.