The Portuguese were among the first Europeans to begin to explore the regions around the Congo River in the late 1400s and to make contact with the indigenous people of the areas. It was not until the 1800s, however, that other European nations became interested in colonizing the area. By that time, the Portuguese had already established themselves in the area to the south of the Congo River, which is now known as Angola.
Natural fortifications also helps defend a country. For example the Alps in Italy defends them very well. Rivers can also be a defensive position a country can fall back to. So no, fortifications and defenses arent the only thing. Nature is able to help too.
