Wilhelmstein

This fortress I visited as a spotty youth on German exchange to Hanover in the 70s. It was built between 1761-5 and was built by getting fishing boats delivering stones to the site. Now a museum. Pictures and below text from the Wiki

This is an artificial island in lake Steinhuder Meer with an area of 12,500 square meters, located in the Hanover Region, Northern Germany. The island was created in the 18th century as a fortification by Count William of Schaumburg-Lippe, ruler of this small German state. Since then the island hosts the Festung Wilhelmstein, the Wilhelmstein Fortress. Today the island, close to Hagenburg, is a popular destination for tourists. It can be reached by so called 'emigrants boats' from Steinhude and Mardorf.