[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.

[2] The island is unglaciated with two lakes between two mountain ridges.

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