Castle Isleif was located in the northeastern part of Caithness, Scotland, between the villages of John o' Groats and Dunnet, about 2 miles from cliffs overlooking the Pentland Firth toward the Orkney Islands.
The first permanent edifice was probably built on the site around 1000 CE and was likely a wooden structure based on a motte-and-bailey plan, according to archaeological research carried out on the site by Marcus McGonagall I, and later by his grandson, Thorfinn McGonagall.
The name "Castle Isleif," first recorded in 1156, likely reflected the Norse origin of the property's original settlers.
The castle was partially destroyed around the time of the Battle of Tannach during the hostilities between clans Keith and Mackay, and Clan Gunn. It was rebuilt as a stone z-plan tower castle tower castle between 1440 and 1460.
The first known settlers of the area that would encompass Castle Isleif were descendants of Vikings that invaded the Kingdom of Alba in the 9th century CE. It is unknown if the original settlers were magical folk, but by the time they rose to the ranks of Toiseachs to the Earls of Orkney, the magical identity of the property's masters was established.
The best known of the Toiseachs was Einar Thorbjornson, who served Thorfinn the Mighty during the latter's war with Karl Hundason over the rule of Caithness and may have used magic to help secure Thorfinn's victory.
The first mention of a "Mac Conghail" as the lord of Castle Isleif comes from the Historia Gentis Scotorum 'Magicum (ca. 1346). It is likely that the Norse owners of the original Castle Isleif assimilated with their Celtic neighbours sometime after Caithness became a tributary of King of the Scots subsequent to the Treaty of Perth in 1266.
House-elves attached to the McGonagall family lived in the castle from 1635 on.
- Thorfinn McGonagall
- Morna MacLaughlin
- Elisabeth Bones-McGonagall
- Minerva McGonagall
- Einar McGonagall
- Katherine McGonagall
Castle Isleif's grounds included a large informal garden and an old dovecote dating from the 16th century.
In modern times, the below-grounds level of the castle was used as a wine cellar and cool storage area for food.
The ground floor included the main entryway and entry hall, as well as the kitchen and servants' (later house-elves') quarters.
A large wooden staircase led to the upper floors of the castle.
The first floor housed the public rooms of the house, including a music room, library, and a great hall, which was partitioned off to become a smaller dining room and a drawing room, later used as Thorfinn McGonagall's study.
The second floor comprised several bedrooms and, originally, two solars. In the 1860s, Drusus McGonagall renovated the second-floor rooms, dividing them into three suites: two smaller bedrooms with attached dressing rooms, and one master suite with bedroom, dressing, and sitting rooms.
In the early 1900s, Marcus McGonagall II eliminated the two garderobes on the second floor and installed modern Magical plumbing throughout the castle, adding private baths to the bedrooms on the second and third floors.
The third floor included several bedrooms, one of which was used as the nursery and schoolroom in the modern era.
One of the bedrooms became known as the "Green Bedroom," after 10-year-old Einar McGonagall placed a charm on it that made anyone who entered it feel inexplicably nauseated as a prank on a tutor he particularly disliked. Einar's father, Thorfinn, lifted the charm when he discovered it, but the family thereafter referred to the room as the "Green Bedroom."
Castle Isleif was protected by powerful charms, including blood wards, that prevented unauthorised access.
Like Hogwarts, it was charmed to be largely unnoticeable by Muggles. Any who wandered too close would be seized by a feeling of danger. Apparition into or out of the house and nearby grounds was impossible (except for house-elves); however, witches and wizards could Apparate to grounds within a mile of the house.
In 1920, Thorfinn McGonagall had Castle Isleif connected to the British Floo Network to allow his ailing father easier access to the Edinburgh and London galleries and libraries he enjoyed, as Apparition from the grounds had become too taxing.
The family routinely provided Portkeys for the use of visitors who preferred not to (or could not) Apparate.