The Clamp Academy (CLAMP学園, Clamp Gakuen) is a fictional school/functioning community in the southern area of Tokyo built in the shape of a pentagram enclosed within a circle. The Academy was built by the Imonoyama family with the dual purpose of providing some of the most talented minds in the world with an educational setting and sealing the Shinken within the center of the pentagram which the school itself forms.
The unusual layout for the Campus resulted from an agreement between Toru Magami and the former director of Clamp Academy for the school to contain and conceal the Sacred Sword, even though the school was nearly half-built and most of the buildings (built and about to be built) needed to be moved. Every aspect of the Academy's design, from the layout of the buildings to the architecture to the infrastructure, is actually carefully plotted to help form the dynamic spell capable of sealing the Sacred Sword. Par example: the trains which run between the five cardinal points of the Campus's pentagram and along the perimeter of the circle run only in one direction and at a speed which is only slightly faster than a person is able to run. The constant retracing of the pentagram and circle by way of commuter train is but a single aspect of the seal.
Although it never comes under attack by the Angels (indeed, it seems that the Angels—excluding Fuma, perhaps—cannot transgress the boundaries of the Academy), Clamp Academy seems to act as a protective zone much in the same way as the Spirit Shield. After Fuma Monou's awakening as the Kamui of the Dragons of Earth, Nokoru Imonoyama provides the Dragons of Heaven with all of the resources at the disposal of Clamp Academy.