Armitage was born Ella Sophia Bulley in Liverpool, the second daughter of Samuel Marshall Bulley, a cotton merchant, and Mary Rachel Raffles, daughter of Congregational minister Thomas Raffles.

Armitage – along with John Horace Round, George Neilson, and Goddard Henry Orpen – proved in a string of publications that British motte-and-bailey castles, which had previously been assumed to be of Anglo-Saxon origin, were not constructed until after the 1066 Norman conquest of England.

Her book The Early Norman Castles of the British Isles is considered a seminal work on the subject.

[1] She also contributed to volume 2 of the Victoria County History, of Yorkshire, writing on ancient earthworks.

[6]: 332 She was also known as a hymnwriter,[7] apparently saying "I believe I was intended by nature for an archaeologist, but life has made me a hymn writer, and I shall be content to be known as such when my archaeology is forgotten.