The castle was on the Grogarnshaupt, the northern tip of Grogarnberges. The 30 meter high sea cliffs provided protection on three sides. In the south there were two 100 meter long stone walls with palisades. From the castle you can see the other great castles of this region. Archaeological investigations of the outer ramparts turned up arrowheads from the late Vendel Period (550-800 AD), which provide a clue as to the dating of the castle. Also found were a number of wooden posts, probably formerly part of the palisades, which dated to the year 740. The castle has both an outer and inner wall.
On the isle of Gotland 82 such castles are known. Most measure less than 800 m² for interior surface. Grogarnsberget, however, measures 450 m × 100 m (45,000 m²), making it one of the two largest towns on the island. House foundations from the Roman Iron Age - the migration period - were found inside the castle, unlike on other castle hills of the island.
- Aleksander Loit (ed.): The contacts between the eastern Baltic and Scandinavia in the Early Middle Ages. International Conference, 23-25th October 1990, Riga. Centre for Baltic Studies, Stockholm 1992, ISBN 91-22-01503-5, p be 53
- Nordic Archaeological Abstracts, Viborg 1984 ISSN 0105-6492 0105-6492, p 125
- "Grogarnsberget". European Environment Agency. Retrieved 2012-01-15.
- "Grogarnsberget Natura 2000" (PDF; 307 kB). Länsstyrelsen Gotland (in Swedish). Retrieved 2012-01-31.
- "Grogarnsberget Nature Reserve". Länsstyrelsen Gotland (in Swedish). Retrieved 2012-01-31.
- Marita Jonsson, Sven-Olof Lindquist: culture Almqvist and Wiksell leader Gotland, Uppsala 1993, ISBN 91-88036-09-X..
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- "Information board" (PDF; 1.5 MB). Länsstyrelsen Gotland (in Swedish). Retrieved 2012-01-31.