[1] Administratively it is part of the Northeast Greenland National Park.

Castle Island is located in Saint Andrew Bay, east of Hendrik Island, in the mouth area of Sherard Osborn Fjord.

It is separated from Hendrik Island by a 5 kilometres (3 miles) wide sound.

[3] The peak in its eastern end reaches 660 metres (2,165 feet).

The sea around the island is rarely free from ice.