Koshu Kaido, one of the five official highways laid by Shogun Ieyasu in Edo (present-day Tokyo), which is now known as Route 20 or Shinjuku-dori, runs west from Hanzomon, the rear gate of the Imperial Palace, formerly Edo Castle, heading straight toward the Province of Koshu (now Yamanashi Prefecture).
Ieyasu planned this highway, not so much for traveling as to secure the shogun's safe retreat in the event of his castle's fall. The road was therefore lined with multifold, subtly laid-out layers of defense.
The emergency provisions began with the protection of the rear gate, assigned to Hattori Hanzo (1542-96), a distinguished lancer who had fought many battles for Ieyasu before he became shogun. After being assigned to Edo Castle in 1590, Ieyasu charged this warrior, famous for his integrity, with guarding the castle's rear quarters and entrusted him with a band of 200 ninja -- secret agents who practiced the art of disguise and secret entry. Hence the origin of the gate's name to this day: Hanzomon or Hanzo's Gate.