(Old Guards House)
Working place for (former)Ninja who used to protect the Edo castle.
There are several guard houses(known as checkpoints) from ohtemon to the main edo castle building.
The most strict checkpoint was called “hundred-guards house”.
(Hyakunin means 100 persons.)
At that time, Negoro, Iga, Kōka and Nijugoki were guarding the castle as a last guard house.
If you know some Ninja history, you know Ieyasu hired many Musket Ashigaru from Iga, Koka region when he started new Shogunate government.
They were given Dōshin class which was low rank Samurai.
So they were no longer Ninjas but Tokugawa retainers.