After transferring his capital to Delhi from Agra in 1638, Shah Jahan commenced the construction of Shahjahanabad, and a little later, on the 16 April, 1639, he also laid the foundation of his citadel, Lal-Qila or Red Fort. It was completed after nine years in April 1648. The entire fortis said to have cost about one crore of rupees, half of it on the palaces. The Red Fort, so called because of the red colour of the stone largely used in it, is octagonal on plan, with two longer sides on the east and west. On the north the fort is connected by a bridge with Salimgarh. The bridge built by Jahangir to connect the Salimgarh with the main land was demolished by British to make way for the present railway bridge. Salimgarh, originally protected by nineteen bastions was built by Islam Shah Sur (A.D. 1545 -1554) also known as Salim Shah Sur of the Sur dynasty. It stood on an island close to a west bank of Yamuna. Recent excavations have proved that it was an ancient site yielding Painted Grey Ware, a pottery associated with sites mentioned in Mahabharata. Salimgarh served purpose of State prison where Murad Buksh and Suleman Shikoh were kept by Aurangzeb. In A.D. 1756 Shah Alam was also imprisoned here after being blinded. The barracks were used by British for keeping in detention the war heroes Shah Nawaz Khan, Prem Kumar Sahgal and Guru Baksh Dhillon, heroes ofIndian National Army. It measures about 900 m by 550 m, with its rampart walls covering a perimeter of2.41 km and rising to a height of 33.5 m on the town side and 18 m along the river.
Outside the ramparts runs a moat, originally connected with the river. The palaces lie along the eastern side of the fort, while two imposing three-storeyed main gateways flanked by semi-octagonal towers and consisting of several apartments are located in the centre of the western and southern sides and are known as the Lahori and Delhi Gates respectively.