The first Vasknarva order castle (Neuschloss) was founded in 1349 on the northeastern border of Old Livonia. 1427–1442 a new castle (Vastne-Narva) was built, which became the centre of the vogtei of the Livonian Order. The castle was wracked in the Livonian War. Until the Great Northern War it was a fort of great importance, commanding the mouth of the Narva River. It has been known in Russian chronicles either as Syrensk or Syrenets. According the folklore St. Olga of Pskov narrowly escaped drowning when crossing the Narva rapids at Syrenets. Nowadays only parts of 3 meters thin walls have survived, mainly on the northern side.
Reference: Wikipedia
Bomarsund fortress and garrison was built in 1832-1854 by Russian Empire to defence Åland Islands against enemies. British and French fleet attacked against it during the Crimean War in 1854. After a week of fighting the British stormed the remaining fort. Anglo-French forces destroyed it totally after battle. In the Treaty of Paris 1856, the entire Åland Islands were demilitarized, which is a status that has been preserved until this day.
Nowadays there are some ruins left and fortress area is open for visitors.