Flagstaff Tower, located in the Civil Lines area north of the Red Fort, was the site at which the European survivors of the Uprisings of 1857 gathered on 11 May 1857.
Flagstaff Tower is a one-room, castellated tower, built around 1828 as a signal tower. It is located in Kamla Nehru Ridge near the North Campus of Delhi University in Delhi. It was here that many Europeans and their families sheltered on May 11, 1857, during the Siege of Delhi by the rebels at the beginning of the Indian rebellion of that year, waiting for help to arrive from nearby Meerut.  Built by the British Indian Army, the building was part of the British cantonment and was used as a signal tower. Before forestation started in 1910, the area where the tower was built was the highest point on the ridge and was mostly barren, covered with low-lying shrub. Today it is a memorial and "protected monument" under Archaeological Survey of India.
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Some modern photos: First Second
Source: British Library