Short answers, pulled from the story.
Allahabad Fort was built by the Mughal emperor Akbar in 1583. Akbar chose the site at the confluence of the Ganges and Yamuna rivers, a location of deep religious significance for Hindu pilgrims, and named the fort Illahabas, meaning "blessed by Allah."
The Akshayavata is a sacred tree enclosed within the walls of Allahabad Fort. Pilgrims historically came to it to end their lives in search of spiritual salvation. Akbar's engineers deliberately incorporated it into the fort's layout.
The Company first garrisoned the fort in 1765 under the Treaty of Allahabad, signed after the Battle of Buxar by Robert Clive, Shah Alam II, and Nawab Shuja-ud-Daula. After decades of contested ownership involving the Nawabs of Awadh, financially pressed Nawab Saadat Ali Khan I finally ceded the fort to the Company in February 1798.
Mughal prince Salim, the future emperor Jahangir, revolted against his father Akbar in 1600 and established his own court inside Allahabad Fort. The rebellion was short-lived; Salim commanded very little territory and was reconciled with his father shortly after.
Allahabad Fort is the largest fort ever built by Akbar. According to historian William Finch, between 5,000 and 20,000 workers of different trades built it over a period of forty years. The fort features three galleries flanked by high towers.
According to Prayagwal Brahmin tradition, the fort's foundations kept sinking in the sand until a local Brahmin voluntarily sacrificed himself to allow construction to proceed. In exchange, Akbar granted the man's descendants exclusive rights to serve pilgrims at the Triveni Sangam.