Suomenlinna is a historic sea fortress located on a group of islands off the coast of Helsinki, Finland. Built in the 18th century by the Swedish to protect against Russian expansion, it is now a UNESCO World Heritage Site. The fortress features impressive walls, bastions, and museums, showcasing its rich history. Today, Suomenlinna is a popular tourist destination, attracting visitors with its scenic views and cultural events. The islands are accessible by ferry, and visitors can explore walking paths, cafes, and art galleries. It serves as a reminder of Finland's military history and architectural heritage.