Why does the family build a fort on the edge of the savannah in swiss family robinson

In the Swiss Family Robinson, the family in the story line decided to build the fort in the edge of the savannah in order to protect themselves from harm and to be able to look out for each other as they knew that the pirates or people in living nearby could attack them any time soon. This is the reason why they decided to build the fort on the edge.

To protect themselves from the wild animals on that part of the island. hope this helps