[1][4][5] The castle, or fort, was built on Grogarnshuvud ("The Grogarn Head"), the northern tip of Grogarnsberget.

The 30 m (98 ft) high sea cliffs provided protection on three sides.

Archaeological investigations of the outer ramparts turned up arrowheads from the late Vendel Period (550-800 AD), which provided a clue as to the dating of the castle.

[6] A number of wooden posts were also found, probably formerly part of the palisades, that were dated to 740 AD.

House foundations from the Roman Iron Age – the Migration Period – were found inside the fortifications, unlike any other hillforts on the island.