Merchiston Castle was probably built as a country house, but its strategic position and the turbulent political situation required it to be heavily fortified – with some walls as much as six feet thick – and it was frequently under siege.

During restoration in the 1960s, a 26-pound cannonball was found embedded in the Tower, thought to date from the struggle in 1572 between Mary, Queen of Scots, and supporters of her son, James VI.

In 1772, a year before the sixth Lord's death, the Tower was sold to a relative, Charles Hope-Weir, second son of John Hope, 2nd Earl of Hopetoun.

Shortly after being let to Merchiston Castle School it was considerably altered with the addition of a castellated Gothic-style two-story extension (see picture) and a basement, which has since been removed.

Edinburgh Napier University has taken out large sections of wall on the northern extension to accommodate a corridor which runs through the Castle to other campus buildings.