There's a bug which causes a player who opens the Samurai castle gate to be warped back to their starting position (of pressing a to open the gate). This happens on either side of the gate which results in no access through it.
The only way to bypass this and enter the castle is to build a ship, and access the castle via the shipyard. This isn't possible since shipyard access isn't a right many ranks have. I hope this is fixed as soon as possible.