Near the gardens of the Taj Mahal stands the important 16th-century Mughal monument known as the Red Fort of Agra. This powerful fortress of red sandstone encompasses, within its 2.5-km-long enclosure walls, the imperial city of the Mughal rulers. It comprises many fairy-tale palaces, such as the Jahangir Palace and the Khas Mahal, built by Shah Jahan; audience halls, such as the Diwan-i-Khas; and two very beautiful mosques.  The fort spreads over an area of about 94 acres of land. At present, there exist more than two dozens of monuments in the Fort.

The Red Fort of Agra is a powerful fortress founded by the Emperor Akbar (1556-1605) on the right bank of the Yamuna; it is placed today on the north- west extremity of the Shah Jahan Gardens which surround the Taj Mahal and form, with them, an evident monumental unity.

This bastioned fortress, with walls of red sandstone rising above a moat, encompasses within its enclosure walls of 2.5 kilometres, the imperial city of the Mughal rulers. Like the Delhi Fort, that of Agra is one of the most evident symbols of the Mughal grandeur which asserted itself under Akbar, Jahangir and Shah Jahan.

The citadel comprises a large number of fairy-like palaces: the Khas Mahal, the Shish Mahal, the octagonal tower of Musamman Burj, as well as reception rooms : Diwan-i-Khas, built in 1637 and Diwan-i -Am, constructed as well, under the reign of the luxury-loving Shah Jahan. One may also note, within the palatial complex, two very beautiful mosques of white marble, the Moti Masjid or the Pearl Mosque, constructed from 1646-1653 by Shah Jahan and the Nagina Masjid built under the reign of Aurangzeb (1658-1707).

All of these monuments mark the apogee of an Indo-Muslim art strongly marked by influences from Persia which already manifested itself in the Timurid art.

Criterion (iii):  This fort is an exceptionally amazing masterpiece that represents the magnificent architecture of primeval epoch.

Source:

https://www.tourmyindia.com/heritage/agra-fort.html

http://asi.nic.in/asi_monu_whs_agrafort.asp

http://www.india.com/travel/agra/