The creation of a psychic defensive fortification is the apex of shielding and when done properly can insulate a practitioner from most any form of magical and psychic assault encountered. At its simplest, a set of nested psychic shields functions as a defensive fortification. Within the safety of the shields, one finds themselves free of the distractions of psychic noise from their environment, hostile encounters, and able to experiment with psychic constructs with out interference from 'external' pressures. The nested psychic shields present a smaller 'space' wherein one might operate. As such, the visualization of the nested psychic shields limits the range of psychic constructs that can be developed within them.
With the addition of mental space between the shields and in the inner area, one creates a psychic construct that allows them greater range of constructs that can be developed and manipulated. This arrangement of shields with mental space between them is known in some circles as a castle. This can be visualized as a series of spheres around the psychic or it can be visualized as a brick and mortar castle with outer bulwarks and a moat. The psychic castle draws off of the skills of hardening one's outer aura and developing an inner space wherein a psychic person might operate.
The development of psychic inner space is commonly encountered with guided visualizations where one is instructed to explore some sort of location within themselves. We have unlimited capacity for developing psychic inner space. This could be argued to be a function of the 'As above, so below' maxim which presents the psychic as a miniature version of the cosmos. With intense concentration and strong visualization skills, it is possible for one to create an entire world within themselves. (It can be argued that authors do this on a regular basis.) World building within, however, can draw away the focus and concentration required to maintain one's psychic defenses. This is why such exercises are done in psychically neutral locations that have protective barriers in place, such as a ritual circle.
The psychic defenses used to construct the castle varies. It is possible to use elemental energies to great success. It is also possible to build one on the basis of pure will. In using elemental energies, the psychic taps the thought forms of the elemental energies and draws power for their shield primarily from them. A will based psychic defense draws upon the creator's willpower and mental energy. Will based psychic defenses are good for short term defense but can prove exhausting to the psychic utilizing them.
A combination of shields with space between them is also useful for defensive measures that act as passive resistance to psychic assault. Charging the space between shields with unpleasant emotions or seeding them with ties to painful experiences is an effective way to push back against the person psychically assaulting you. This works upon the principle that if they can send information when there is contact with you, then you can send information back. Thus, the person attempting to push through your psychic shields will not only encounter the resistance of the shields themselves but the effects of what you have placed between them. This can range from sensations of mental discomfort to sensations of physical discomfort. It can also include vivid experiences of negative memories or thought forms that the defender has experienced, depending upon the force of the experience attached to the space and the receiver's attunement to the sender.
Part of the appeal of a psychic castle defense comes from the simple fact that its function is to protect the psyche of the defender. Another reason why psychic castles are used is to create a 'safe space' where the person who has developed their castle can take refuge from environmental influences. This 'safe space' functions on multiple levels. It serves as a location where the psychic in question is insulated from external psychic stimuli. It is also a mental location where one can take refuge from distressing mental stimuli, such as a traumatic memory.