Maasi medieval fort-castle was built with the forced labour of islanders. That's how the ruling Liivi order punished indigenous inhabitants for the uprising, which had destroyed orders previous stronghold. Seaside fort-castle was undefeated until blown up by Danes. 8m walls that survived the destruction have become a landmark.
Good to know: On August 11th, 1576, Danish king Frederik II ordered Maasi stronghold destroyed, it has been in ruins ever since.