Why did the doctor decide to return to the ship and what did he learn? What was his next course of action?

In Chapter Sixteen, Dr. Livesey discovers a stockade while exploring the island. The stockade consists of 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. While on the island, he (like Jim) hears the death of Alan, and he supposes that Jim has been killed. After their expedition, Hunter and Livesey return to the Hispanolia, and Livesy tells the captain of his newly formulated plan.