Masugata Mon, The Death Box Gates

Secrets and Designs of The Samurai Castle Unbreachable Gate

To bring down a samurai castle, you needed a ratio of at least three attackers to one defender. Around a main castle were numerous smaller castles — satelite castles if you will. Their job was to defend the main castle. Attackers would have to bring down the smaller surrounding castles first, or risk being attacked from behind. Splitting your large army into smaller units to attack these castles weakens your front lines. Then logistics become a problem, you have smaller groups, all separated, making supply and communications difficult too. This gives the main, and smaller castles a chance to fight back and destroy smaller groups of enemy in unfamiliar territory. Attacking these smaller castles first give the main castle time to prepare as well, meaning there was no such thing as a surprise attack.