The Frontier
The Frontier is a massive defensive structure built by the Star League to defend themselves from attack by the Kodan. The entire structure is a defensive wall, similar in many respects to the famous Great Wall of China.
Specifications
The Frontier consists of a massive network of energy barrier generators that project an impenetrable wall that prevents any type of attack by the forces of the Kodan Empire. The generators are positioned inside the barrier they create, rendering them immune to attack from the outside. The barrier stood for unspecified generations until Xur -- a Rylon and son of the Star League president -- defected to the Kodan, taking with him the secret of penetrating the energy barrier.
From the map displays, The Frontier appears to surround Star League space completely, or at least is so large that the Kodan can not simply fly around it due to fuel limitations or other constraints.