It doesn't matter what side you start on but preferably you start with the Japanese.

You run over to the bridge and hop on the rail, and run and jump towards the lining of the wall and the ground, and you should go straight through and under the map.

After you do that, run towards the woods and walk along the outside of the castle wall, until you come to a lip in the wall.

Once you're there crouch and go under that lip and walk up the mountain, and from there you will be able to snipe anyone on the map below.