Suomenlinna, also known as the Fortress of Finland, is a sea fortress and UNESCO World Heritage site located on an island in Helsinki, Finland. It was built in the mid-18th century by Sweden to protect the city of Helsinki against the Russian Empire.

The fortress consists of six islands, linked by bridges and ferry services, and includes various fortifications, barracks, and other structures. Suomenlinna played a significant role in the Russo-Swedish War of 1788-1790, and later served as a garrison for the Russian army until Finland gained independence in 1917.

Today, Suomenlinna is a popular tourist destination and a living community with about 800 residents. It also houses several museums, including the Suomenlinna Museum, which provides an overview of the fortress's history and development, and the Ehrensvärd Museum, which showcases the life and work of the fortress's founder, Augustin Ehrensvärd. Visitors can explore the fortress's various structures and take guided tours, and there are several restaurants and cafes on the islands.