Gustavus Nicolls was an officer in the United Kingdom's Royal Engineers - a corp of specialized Army officers who were assigned to build or destroy fortifications, bridges, canals, and other structures.
Nicolls spent a total of 27 years of his service career in Canada, during three hitches, from 1802 to 1837, and was responsible for, or played a role in, the design and construction of almost all the fortifications built during this time, including the Halifax Citadel, the Quebec Citadel, Fort York, Fort Henry and Fort Erie. He also designed and constructed what is believed to have been the first lighthouse on Lake Ontario.
According to Parks Canada Nicolls service in North America, when his contemporaries were serving in the Peninsular campaign, under Arthur Wellesley, the Duke of Wellington, slowed his promotion. In particular, he and Sir James Carmichael Smyth had been contemporaries at the Royal Military College, but Smyth, who served under Wellington, was a General and a Baronet, when he was Lieutenant Governor of Nova Scotia, while Nicolls was still just a Lieutenant Colonel.
Nicolls married Heriot Frances Thomson, in Halifax, in 1812. His first son was born there. Four more sons were born in the United Kingdom, and Guernsey, during the years 1816 and 1825, when he was between assignments and on half-pay. Jasper Hume Nicolls, the son born in 1818 in Guernsey, settled in Canada
- Stephen Otto (April 2016). "Who Was Gustavus Nicolls?". The Fife and Drum 20 (1). Archived from the original on 2020-03-31. https://web.archive.org/web/20200331132325/http://www.fortyork.ca/images/newsletters/fife-and-drum-2016/fife-and-drum-mar-2016.pdf#page=5. Retrieved 2021-07-05. "Between 1802 and 1837 Gustavus Nicolls of the Royal Engineers served twenty-seven years over three tours of duty in British North America working on most of the major forts here."
- John Joseph Greenough (2006-10-24). "The Halifax Citadel, 1825-60: A Narrative and Structural History". Parks Canada. Archived from the original on 2021-06-24. https://web.archive.org/web/20210624195322/http://parkscanadahistory.com/series/chs/17/chs17-1i.htm. Retrieved 2021-07-05. "He missed the opportunities afforded to officers who had had the good luck to serve in the peninsular campaigns and at Waterloo, with the result that he was still a colonel in the Royal Engineers — a mere major in the regular army — in 1825."