There are two types of Defensive buildings: Beachhead Fortifications (under the «Fortifications» tab on the «Construction» menu) and Improvements (under the «Improvements» tab on the «Construction» menu). To purchase Defensive buildings, you need Rubies and Resources. Your Beachhead is the colored perimeter standing between your Haven and the sea. You can build Walls, Towers, Gates, Cannon, Crow's Nests, and Harpoon guns along this line to improve the defensive capability of all Crew within your Haven. Better Fortifications will become available as you make more advanced Discoveries .