This movie is based on a historical event. The siege of Oshi castle took place in 1590 in what is now called Gyoda City in Saitama.
When the dykes are being constructed broken clay pottery figures can be seen intermingled with the clay. These clay pottery figures (haniwa) were probably from a kofun-tomb that had been demolished to help build the dyke. This area is famous for its kofun-tombs some of which still survive and can be visited nowadays by tourists.
The siege happened in June which is rainy season in Japan, this may have been another reason to use water to besiege the castle.
Kaihime became an iconic figure in Edo Japan. Tales were told of her fighting in the siege which may have been fictional. At the movie it is said nothing is known about her old age but one legend/theory says she was at the siege of Osaka and escaped to a nunnery in Kamakura.
WILHELM SCREAM: approximately at 68 min, when a musket volley is fired at close range against the infantry troops in red armor.