It is heavily fortified, surrounded by a number of garrisons and four cannon bases, containing large cannons that were invented by Da Ji. The castle itself is surrounded by a moat of lava, making it so someone can only reach it by four small drawbridges, creating a difficult path for someone before they can even reach the inner keep, where Orochi waits to battle them. This doesn't take in the serpent king's enormous army, which defends the castle and frequently includes Lu Bu as its final line of defense.
After Orochi's first defeat, Kiyomori Taira uses Koshi Castle as the site of the serpent king's resurrection in Warriors Orochi 2. However the castle didn't come into play as much during the game.