However, when the siege was decided Nobushige built a (crescent-shaped) branch castle made of dirt called the Sanada-maru (Sanada Barbican) outside Tamatsukuriguchi and the south of Sannomaru (outer part of the castle) which was the weak point of the Osaka-jo Castle, and provoked the Tokugawa side using troops with firearms, and gave the advance party a black eye.
しかし籠城の策と決すると、信繁は大坂城の弱点であった三の丸南側、玉造口外に真田丸と呼ばれる土作りの出城（三日月形）を築き、鉄砲隊を用いて徳川方を挑発し先方隊に大打撃をあたえた。