Osowiec Fortress (Russian: Крепость Осовец) - a fortress from the second half of the 19th century, located in the Osowiec-Twierdza settlement belonging to the Goniądz commune in Moniecki County in Poland, known for its 6.5-month defense during World War I. At the beginning of the 20th century, it was considered one of the most modern fortresses in the world. It was never conquered.