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 destroyed by Danes. The fortress was blown up in 1576 by the Danes in an attempt to forestall the Swedish invasion and nothing was done for the next 300 years. 8m walls that survived the destruction have become a landmark.
References:House of the Blackheads (Melngalvju nams) is a building situated in the old town of Riga. The original building was erected during the first third of the 14th century for the Brotherhood of Blackheads, a guild for unmarried German merchants in Riga. Major works were done in the years 1580 and 1886, adding most of the ornaments.
The structure was bombed to a ruin by the Germans June 28, 1941 and the remains demolished by the Soviets in 1948. The current reconstruction was erected from 1995 to 1999. Today the House of Blackheads serves as a museum and sometimes concert hall.