North Slope Mountain is a large landform which rises above the land north of Border Town and Longsong Stronghold. It is closely connected to the Impassable Mountain Range and has a mine in its interior. Later it is removed from the ground, in the same way of the Deity of Gods, to form the 'Eleanor Skycruiser', a floating battleship.
It existed for a long time as merely a part of land without much practical use aside from protection from demons and demonic beasts until ore was discovered inside it, due to the Witch Federation's search for God's Stone of Retaliation veins. It stayed abandoned after the Union's fall until the eventual formation of Graycastle and later formation of Border Town for its exploration.
North Slope Mine was re-discovered after the establishment of a warning outpost at the western border of the Kingdom of Graycastle. To excavate the ores found there, the outpost was turned into a small town called Border Town and ore mining became the main source of income for the local population.
In the beginning of the story, it is only used for its ore extraction, but when a God's Stone of Retaliation vein is found within some research is done near it, especially after Agatha joined the Witch Union. After the acquisition of a Mother of Soul through Eleanor, Graycastle started the 'Heaven Plan' which eventually lifted the entire North Slope Mountain, North Slope Mine included, forming humanity's Deity of Gods. The lifted Mountain was now called 'Eleanor Skycruiser' and had as its main mission patrolling Neverwinter's surroundings until eventual combat occurred and its use as a floating battleship and stronghold was revealed. For Eleanor the floating stronghold acted as her body and the Mother of Soul's tentacles acted as additional perceptive organs improving her control over the the mountain. 
Due to the lack of time, the island didn't have a complete electrical lighting system installed, to the extent that even the resting places were drilled out of a wall. Soraya added a soft layer and isolated the moisture of the underground chambers and made the place into a bedroom. The furniture within were ordinary, with simple wooden tables and cabinets with the mattresses laid directly on the floor. Since all the magic stone illuminations were used for the factories rushing to produce goods, candle light was used on the residential areas.
Aside from the privacy of having a room per person, there were even standalone toilets. There were even ventilation shafts that provided airflow directly from outside the floating island into the chambers, preventing the rooms from being too stuffy or humid. There was a shared reading area and a warm public bath (heated by a magic powered boiler) at the end of the caves as well, enough to satisfy the witches' daily use. 
It is able to project a visualization of the outside using the magic power core, and to fire machine guns automatically (through Eleanor's calculation and telekinetic firing). It apparently moves at around 3 km/h (250km in 3 days) and is the only thing capable of switching between defense and offense seamlessly. 
- Chapter 16-17
- Chapters 272-273
- Chapter 1443
- Chapter 1445-1446
- Chapter 1455
- Chapter 1456
- Chapter 1457
- Chapter 1459