Parts of the ruins of the monastery remain on the island, but much of the stonework was used in the expansion of Akershus Fortress in the 17th century.
When Denmark-Norway became involved in the Napoleonic war, two cannon batteries were built to defend Oslo and Akershus Fortress in 1808 (again using the monastery ruins as a quarry).
It was used by Director of Armory Ole Herman Johannes Krag, co-inventor of the Krag–Jørgensen rifle in the late 19th century.
[citation needed] The island has an interesting geology, being composed mainly of Ordovician and some Silurian mudstone and limestone.
The transition between rocks of the two periods is exposed on the south-eastern tip of the island, illustrating the sudden marine transgression that marks the shift between two important parts of Earths history.
The sequence has been affected by both folding and thrusting as part of the Caledonian orogeny with the strata lying vertically or in some case becoming overturned.
The mostly sedimentary geology has given the island a very fertile soil, giving it Norway's highest biodiversity found in such a small place.