To disable the alarm in the lighthouse without raising the alarm in The Tale of Ryuzo in the Ghost of Tsushima get into the fort through the gap in the wall. Inside, assassinate the guard standing near the pelt. Keep going to the right in front of the house and drop down just past the tent where the fence is broken.
Go towards the lighthouse and go around it to the right and assassinate the lookout.
Keep going around and into the tower from behind and climb the ladder watching out for the guard on the second floor. Get to the top and disable the alarm.