In part four of Treasure Island, why did the doctor decide to move his men to the stockade?

Exploring the island, Dr. Livesey discovers a stockade, a group of logs that would hold a great number of people (about 40, according to Dr. Livesy). More significantly, whoever controls the house would control the entire island, because of it's vantage on a hill. Another significant advantage of the house is that there is a source of freshwater, of which their is a shortage on the island.