On 4 September 1390, Knyaz Feodor was commanding defence of one of the Vilnius Castles during its siege by the Vytautas the Great, Teutonic Knights and "knight guests" from almost all of European states during the Lithuanian Civil War (1389–1392). The Crooked Castle was captured and its commander, Karigaila, brother of Jogaila, was killed. The Upper castle, with Polish staff and lower, commanded by Feodor, were rescued.