Construction of Château de Mérens castle dates from the end of the 13th and beginning of the 14th centuries. It was altered in the early 17th century.
The hub of the castle corresponds to the original Gascon structure; the south west square tower belongs to this period of construction. At the start of the 17th century, the castle was furnished with a new system of defence, including a round walk. At the same time, windows were added and the internal layout altered. In the west elements of the mediaeval enceinte can be seen in the wine cellar. Works carried out between 1604 and 1613 concentrated on providing windows and on doubling the height of the east façade. The castle was later converted for agricultural use.
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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.