The circle occurs very frequently in yantras and is derived principally from the motion of the revolution of planets. It symbolizes wholeness or totality and represents the principle which has no beginning, and no end, for example time. A perfectly symmetrical entity, equidistant from the center at all points; it indicates the realm of radiation that proceeds from the One center. In other words, a circumscribed field of action.
When used in a yantra, a circle is normally placed within a square pattern, described next:

The square is the fundamental format of most yantras. There is a significant, well thought out logic behind it. The phenomenal world extends into four directions. These four directions represent the totality of space, and they bind the earth in order. The square too is the simplest and perfect manifestation of the number four, by virtue of the four perfect, equal lines bounding its form. Hence it is visualized as the perfect symbol to denote the terrestrial world. This mundane, physical world is the one which must be transcended by spiritual practice.
The square pattern has four gates, one in each of the cardinal directions. They are known as cosmic doors because it is through them that the aspirant symbolically enters the yantra. They represent the passage from the earthly realm to the 'inner', sacred space of the yantra. These gates are an initiatory threshold which simultaneously opposes the phenomenal and embraces the noumenal. It is further believed that these gateways themselves are guarded by divine forces which protect the sacred precinct within, from negative and disintegrating forces.

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