To sneak into Lord Shimura’s Quarters in Heart of the Jito in the Ghost of Tsushima jump into the lake, swim towards the waterfall and climb onto the rock.
Follow the cliff around until you get into the house. Go to the front left of the house and leave it.
Go down into the gardens and follow the right wall all the way to the other end. Exit the gardens and stay left to climb the ladder and go into the castle.
Go up the ladder to the left and out of the window. Follow the roof around and climb back into the castle when you find another broken window and climb up the ladder. Go through the door and also the door on the other side of the room. Climb back out of the castle and follow the roof around and climb the ladder. Climb up the second ladder and get to the top of the castle. Go down the ladder into Lord Shimura’s quarters.