Fort I Centralny

Fort I Centralny was Osowiec's main defensive position. Its perimeter measured about 2.6 kilometres, enclosing barracks, a military hospital, stores, officers' houses and a church. It was both a fighting position and the setting for everyday garrison life. The fortress stood at a narrow section of the Biebrza valley, where marshes restricted the movement of large forces. Its forts protected a crossing and railway against attack from East Prussia, using the terrain as part of the defence.

On 6 August 1915, German forces used poison gas at Osowiec. The counterattack by surviving Russian defenders became known as the Attack of the Dead Men. The garrison remained at the fortress until its evacuation later that month.

Russian forces left on orders prompted by the wider situation on the front, destroying some buildings before withdrawing. The defence in 1915 thus ended in evacuation rather than surrender following a German assault. Fort I is now part of the complex closed to visitors.