The Water Transportation in the Kanto Plain and the "Piracy" there (6)

     Almost a millennium ago, the Boso Peninsula was still almost an island, and the most part of the Kanto Plain was marshland.  In the eastern part of the marshland, there ran Tone and Watarase Rivers, meeting and branching one another here and there from time to time, into Edo Bay.  In the western part of the marshland, there flew Kinu River into Katori Sea, forming a large flood plain.  The watershed between the 2 parts of the marshland was so low that they used to be connected to each other through a wetland or a river.  Let me call the 2 parts Tone-Watarase river system and Kinu river system.

     Sekiyado Castle was in Tone-Watarase river system, and Mizuumi Castle was in Kinu river system.  It is presumed that, sometime in the medieval times, the direct water transportation between the 2 river systems became virtually impossible.  They had to transship their cargo from boats to horses and vice versa at Sekiyado and Mizuumi.

     Sekiyado Castle had such structure as if to block a branch river of Tone-Watarase river system.  The castle was so important that Hojo Ujiyasu (1515-1571) appreciated it, “To occupy the castle is as valuable as to win one province.”  As its name suggests, there must have been a checkpoint (“seki”) to guard and exploit merchants and inns (“yado”) for their convenience.

     Mizuumi Castle, on the other hand, was along the lakeside of Kinu river system.  “Mizuumi” literally means a lake.  The castle was surrounded with Lake Mizuumi in the east and the south, and with Lake Kusakabe in the north-west.  It was on a bank-like low hill.  The west side of the castle used to be called Yanahara, and there had been a port town with 3 temples even before the Yanada Family’s ruling.  In other words, the castle was built to guard and exploit the port town.