Suomenlinna Castle is a fortress situated in Helsinki. This is a historic site having its own military museum and even a library. This castle is a major monument with military architecture to it and is almost 260 year old fortress. It has been preserved intact till date as it was used for military purpose and at present, it is a part of the world heritage. In 1991, it was included in the UNESCO’s World Heritage List. Suomenlinna is also an inhabited sea fortress which was built on six islands.