Constructed in 1826, this small guardhouse was originally a brig to hold arrested soldiers until trial. During the Soviet era it was used as offices for various institutions, including a library and a registry office. Today it belongs to the Kostroma Museum Reserve and houses three separate exhibits on military history, with displays of weapons and uniforms (from both Russia and other countries) from the medieval era to modern times.
If you don't want to buy a joint ticket, single-exhibit tickets are sold for R80 (kids get in free).