Kangdez means "Fortress of Kang" and refers to a mythical, paradise-like fortress in Iranian folklore. In Ferdowsi's Shahnameh, Kangdez becomes Gangdez.
The name appears related to Kangha mentioned in the Avesta's Yasht 5.54, the Aban (Ardvisur) Yasht. Antar Kanga is part of a list of mountains in Yasht 19.4.
The Middle Persian Pahlavi texts (Dinkard 7.38; Bundahishn 32.5; Dadestan i Menog i Khrad 27.57; Zand i Wahman Yasn 7.19-20) mention Kangdiz as being founded by Siyavakhsh (Siavosh in the Shahnameh). In the Bundahishn (33) and Dadestan i Denig (90.4) Kangdiz was conquered by Kay Khosrow. In Pahlavi Zoroastrian eschatological works, Kangdiz is the abode of Peshotan (Chitro-maino), son of King Vishtasp, and Khwarsheed-chihr (Khursheed-chehr), son of Zarathushtra, who will gather their righteous army there before the final battle against Ahriman and his creatures (Bundahishn 29.6, 33.28; Dinkard 7.5, 12; Zand i Wahman Yasn 7.19-20). In Dinkard 9.15 the previous information is ascribed to the lost Sudgar Nask of the Sassanid Avesta.
In the Dadestan i Menog i Khrad 62.1, the location of Kangdez is described as "Kangdez is entrusted with the eastern quarter, near to Satavayes on the frontier of Airan-vego."
[Note: Satavayes / Satevis / Satevish / Sataves: Av. Satavaesa, a star or constellation. There is considerable contradiction in the Greater and Lesser Bundahishns about the quarter in which it is the chief star. In some instances it is in the western quarter (L. Bund. 2.7 and 5.1). In others it is the chief star of the southern quarter (L. Bund. 13.1).]
In the Shahnameh, Siavosh, having fled from Kay Kavus to Turan, is granted by Turan's King Afrasiab a pleasant piece of land, where Siavosh erects the castle Kangdez. In other Persian texts, the construction of Kangdez is attributed to Kay Kavus, Kay Khosrow and even Legendary King Jamshid. The region around the castle Kangdez is described (in a manner similar to that of the Aryan homeland, Airyana Vaeja, in the Avesta) as being rich in water and game, and knowing neither the frost of winter nor the heat of summer. It is thirty 30 farsakh square in size. The walled city of Kangdez is also called Kang-e Siavosh, Kang-e Siyawakhsh, Siavoshgerd, and Siyawakhshgerd, in different texts. Descriptions of the city include it having handsome buildings with seven walls built with marble, stone, cement and other materials. There are beautiful meadows with flowers within the city walls. Siavosh lived in Kangdez until he was cunningly killed by Afrasiab. When he learnt of his father's murder, Siavosh's son, Kay Khosrow, pledged vengance. When Kay Khosrow ascended the throne of Iranshahr, he launched a series of expeditions against Turan and Afrasiab, who he eventually defeated. Afrasiab fled to Chin (China) and from there sails to Kangdez. Here we learn that Kangdez is a far away land across the seas. Kay Khosrow pursues Afrasiab, puts together a naval force, and sets sail for Kangdez which he reaches after a six-month-long voyage. But the wiley Afrasiab has already fled Kangdez. After staying in Kangdez for a year, Kay Khosrow sails back to Iran through Turan.
According to Arab geographer, al-Biruni, identifies Kangdez with another legendary land Yamakoti in the following statement. "Yamakoti is, according to Yakub and al-Fazari, the country where is the city Tara within the sea. I have not found the slightest trace of this name in Indian literature. As koti means castle and Yama is the angel of death, the word reminds me of Kangdez, which according to the Persians had been built by Kai Kaus or Jam in the most remote East, behind the sea... .Abu Mashar al-Balkhi based his geographical canon on Kangdez as 0 degrees longitude."
The reference to 0 longitude alludes to the concept that Kangdez is considered the centre of the earth.
The geographers who used Kangdez as the prime meridian belonged to what is known as the al-Balkhi school, after Abu Mashar al-Balkhi, known in the West as Albumasar. During the Middle Ages, Albumasar was the most renowned of Muslim astronomer/astrologers in Europe. His theories of historical cycles linked with the planets influenced many European astrologers including Nostradamus whose key work Revolutions was based on such concepts.
The al-Balkhi school placed the meridian in the far East. His followers saw Kangdez as the same as the Indian Yamakoti. In addition to Kangdez, the city Tara / Nara was placed in Yamakoti at the equator.
In al-Qanun al-Masudi, al-Biruni writes that Tara was 90 degrees east of Lanka / Ujjain basically agreeing with the Indian texts about the position of Yamakoti. Again Yamakoti was the same distance from Lanka / Ujjain as the latter was from Romaka (Eastern Roman empire or Constantinople).
Al-Biruni did not accept the equation of Lanka's longitude with that of Ujjain. He thought instead that it referred to the isle of Langabalus, the island of cloves (lavang), which may refer to the Nicobar-Andaman chain. Even though the Indians used Lanka as their own meridian, they usually named Yamakoti first when listing the four quadrants of the globe.
If we accept the Ujjain meridian, the longitude for Yamakoti would be at around 120-122 degrees East longitude which passes through the Indonesia of today.