Garbagriha (sanctum) in a Hindu temple and its symbolism

Big sanctum of Thanjavur Brihadeeshwar temple,
Chennakeshava Temple,
Above image:  It is a small sanctum/ garbagriha. Take note of the fine decorations on the wall above the and around the entrance door. Devotees offering prayers in front of the main  Garbhagriha in Chennakeshava Temple, Belur, which houses the icon of the god Vishnu...............................................

Garbhagriha (in Sanskrit  garbha means  womb and griha means house) / sanctum sanctorum is the most important part of a Hindu place of worship. It is akin to a dark cave in a mountainous area. Jain and Buddhist temples do have a sanctum.  It is here  resides the murti (idol or icon) of the primary deity of the temple. Generally in Hinduism only 'priests' (pujari) are allowed to enter this chamber and have the rights to touch the enshrined idol and carry on the daily puja protocol, etc. Garbhagriha is known as ''karuvarai'' in Tamil  meaning  "womb chamber". The word' karu' means foetus and arai means a room. It is a primordial womb and Moksha (salvation) implies merging with this womb and there is no rebirth

Rooted in Vedic traditions,  a Hindu temple  structure is designed  as to promote a link between  human beings and gods together. It is symbolic of God's residence in Vedic form composing circles and squares, depending on the style  being followed.  In a Hindu temple, you can see the incorporation of all elements of Hindu cosmos - presenting the good, the evil and the human,  besides  elements of Hindu concept of cyclic time and the essence of life. Symbolically a man has to go through different phases in his life time - dharma, kama, artha, moksa, and karma.
The sanctified space in a Hindu temple  is symbolic of the link among  man, deities, and the Universal 'Purusa'. It represents the triple-knowledge (trayi-vidya) of the Vedic vision. Here are found all cosmic elements that are essential for sustenance of life. Here, you will find  fire to water, images of nature to deities, male and female forms, all kinds of fleeting sound and finally eternal emptiness to cosmic energy.  According to  ancient Indian texts on temples: A  temple is a place for Tirtha - pilgrimage, it is a sacred site with unique  ambiance  that reverentially condenses the ideal tenets of Hindu way of life.  The deepa Aradhana - waving of light  in front of the deity  symbolises  the eternal flame or the unfathomable source of energy that keeps the universe going.
Hollow tower above sanctum, Brahadeshwar temple, Thanjavur, TN,  
 Thanjavur big temple inner view of the tower, and lingam below.
The temple is like a human in a lying position with the head forming the sanctum and the feet forming the entrance point, symbolic of journey into self-realization after passing through various phases in our life.  
Sanctum -doors are closed Doddabasappa temple,Karnataka

Thanjavur Shiva linga in natural the sanctum  Divya Darisanam
The sanctum of a Hindu temple has a single entrance, normally facing East direction  to allow early rays of the sun to enter  and dispel the darkness. It has no windows on all the walls around it. In the Dravidian style of architecture, the tower (Vimana) over the garbagriha is small with exceptions like the Thanjavur Big Temple dedicated to God Brihadeeswar (Lord Shiva). Here, it is a tall tower reaching 216 feet made of interlocking granite blocks. So, from inside you can see a hollow space. As for temples of Bihar and northern states, the tower (Shigara) above the sanctum is deliberately a tall one.

Garbhagriha is symbolic  of dark cosmos  that envelops the world  after 'pralaya' (deluge) - total annihilation marking the end of a cycle of creation. The hidden source of light and creation in the darkness is what is called  Brahman  the supreme existence or absolute reality. Mind you, it is formless.  In Hinduism this formless reality takes a form to get embedded in devotees' mind. Coutless deities in the Hindu pantheon are the forms of ''One God''. Depending on the nature of temples that ''form'' is enshrined in the sanctum in accordance with Agama sastras.  The tradition has been that garbhagriha is  seldom renovated, however it is not so in the case of outer walls of garbagriha.  

An  oil lamb is  continuously  kept going  day and night in the sanctum symbolic of driving away the dark evil forces (negative charges).  The sanctum, deity's vahana (mount), Nandi in the case of Shiva or Garuda in the case of Vishnu), the flag-staff  and the entrance gate with gopura (tower) over it, all fall on the same  horizontal  East-West axis. 

Garbagriha is an independent structure and  the deity is set in such a way, it receives the cosmic energy coming down  right through the top of  vimana above. The center of Vimana and the center point of the deity will be on the same axis.

In some sanctums you may find main deity with other sub-deities, ex. Vishnu with his consorts and Garuda. Invariably in most garbagrihas, you will find only one deity. There is only one entrance to the sanctum. In some temples, the procession god(s) -utchavars may be kept inside the sanctum and offerings will be made to them instead of to the main deities (Moolavars). During festive times, utchavar deities are kept outside the sanctum  in a near-by hall (mantab) where the pujas are done as usual. 

The sanctum is deliberately kept free from natural light and  there are two reasons for it: To help the devotees concentrate on the deity and the other possible reason is this most sanctified place on the temple premises is charged with positive energy. The belief is this positive energy will help devotees  gain confidence and face the worldly problems with disciplined and positive mind. The satisfaction of having visited the temple and stood before thedeity will energize him and keep a hold on him. Harboring selfishness, ego, arrogance, etc while on the temple premises, will affect the flow of positive energy and meditation. 'A polluted mind will never see perception in God'.
Garbhagriha  may have a  square, round or oval plan . You can see sculptures or paintings related to the Hindu mythology. Invariably, the inner walls of Garbagriha are plain and devoid of any sculptures and embellishments implying its emptiness.

The temple authorities will not make any compromise on the divinity and sanctity of the sanctum as it is a source of positive energy.