Short answers, pulled from the story.
The Taj Mahal was built by Mughal emperor Shah Jahan as a tomb for his wife Mumtaz Mahal, who died in childbirth in 1631. Construction commenced in 1632 and was completed in 1653, employing more than twenty thousand workers from India, Persia, the Ottoman Empire, and Europe. The chief architect was probably the Persian architect Ustad Ahmad Lahori.
Agra served as the capital of the Mughal Empire from the reign of Babur, who took the city in 1526 after the First Battle of Panipat, until 1648, when Shah Jahan shifted the capital to Shahjahanabad. Aurangzeb moved the entire court to Delhi in 1658, after which Agra declined rapidly.
The Koh-i-nur diamond was presented to Humayun, son of Babur, by the family of the Raja of Gwalior in Agra following the First Battle of Panipat in 1526. The Raja's family gave Humayun the jewel as a token of gratitude after he protected them from plunder.
Air pollution from industrial foundries and vehicles, combined with insect infestations fed by the heavily polluted Yamuna river, have caused the Taj Mahal's white marble to discolour. Agra ranked as the 8th most polluted city in India in a World Health Organization study using data from 2010-2016. A 1996 Supreme Court of India ruling created the 10,400 km2 Taj Trapezium Zone to impose strict industrial pollution controls around the monument.
News of the mutiny at Meerut reached Agra on the 14th of May 1857, and by the 3rd of July some 6,000 Europeans had taken refuge inside the Agra Fort. British forces were defeated in a sally on the 5th of July, and Lieutenant-Governor J.R. Colvin died inside the fort and was buried before the Diwan-i-Am. A relief column under Brigadier Edward Greathed arrived on the 11th of October and repelled a surprise rebel attack in what became known as the Battle of Agra.
The Agra Fort was commissioned by Emperor Akbar in 1565 and took roughly eight years to build. Its red sandstone walls rise 21 metres and enclose a perimeter of about 2.5 km. Key structures inside include the Jahangiri Mahal, the Diwan-i-Am, the Diwan-i-Khas, the Moti Masjid built entirely of white marble by Shah Jahan, the Musamman Burj, and the mirror-inlaid Sheesh Mahal.