Shimabara had once been the domain of…

Shimabara had once been the domain of the Arima lordly family, which had been Christian; as a result, many locals are also Christian.

The Arima were moved out in 1614 and replaced by the Matsukura.

The new lord, Matsukura Shigemasa, had hoped to advance in the shogunate hierarchy and had involved himself with various construction projects, including the building and expansion of Edo Castle, as well as a planned invasion of Luzon.

He had also built a new castle at Shimabara.

As a result, he had placed a greatly disproportionate tax burden on the people of his new domain, and further angered them by strictly persecuting Christianity.

Those affected also include fishermen, craftsmen and merchants.

Even the Dutch, who have a trading post nearby and are anti-Catholic, had been startled at the excessive degree of repression.

These policies have been continued by Shigemasa's heir, Katsuie.

The inhabitants of the Amakusa Islands, which had been part of the fief of Konishi Yukinaga, suffer the same sort of persecution at the hands the Terasawa family, which, like the Matsukura, had been moved there by the Shogunate.

Other masterless samurai in the region include former retainers of Katō Tadahiro and Sassa Narimasa, both of whom had once ruled parts of Higo Province.

The discontented, masterless samurai of the region, as well as the peasants, have begun to meet in secret and plot an uprising; this breaks out in the autumn of 1637, when the local daikan (tax official) Hayashi Hyōzaemon is assassinated.

At the same time, others rebel n the Amakusa Islands.

The rebels quickly increased their ranks by forcing all in the areas they take to join in the uprising.

A charismatic sixteen year-old youth, Amakusa Shirō, is soon chosen as the rebellion's leader.

The rebels besiege the Terasawa clan's Tomioka and Hondo castles, but just before the castles are about to fall, armies from the neighboring domains in Kyūshū arrive, and force them to retreat.

The rebels then cross he Ariake Sea and briefly besiege Matsukura Katsuie's Shimabara Castle, but are again repelled.

At this point they gather on the site of Hara Castle, which had been the castle of the Arima clan before their move to the Nobeoka Domain, but had since been dismantled.

They build up palisades using the wood from the boats with which they had they had crossed the water, and are greatly aided in their preparations by the weapons, ammunition, and provisions they had plundered from the Matsukura clan's storehouses.