PoznaÅ Fortress ( in Polish : Twierdza PoznaÅ , in German : Festung Posen ) was a set of fortifications in the city of PoznaÅ in western Poland , built under Prussian rule in the 19th and early 20th centuries . PoznaÅ Fortress , known in German as Festung Posen ( Polish : Twierdza PoznaÅ ) was a set of fortifications in the city of PoznaÅ ( German : Posen ) in western Poland , built under Prussian rule in the 19th and early 20th centuries .