The building was used primarily for flag signalling. The British military first used Signal Hill, originally known as the Lookout, for this purpose in around 1704. There was a mast at the highest point on the hill, and it would be the job of a signalman to keep an eye out for ships approaching the city. When one was spotted he would indicate by flag what type of ship it was, what country it was from and fire off a peterra so that the men below in the town would know to look towards the hill. Then they would fire the same type of cannon in response. By the end of the 18th century, a blockhouse had been constructed on the site. By the 19th century, the signalling was done mostly for the various merchants who operated out of St. John's. Cabot Tower was the last in a line of signalling blockhouses built on Signal Hill. It was used for flag signalling until 1958.