Complex sentence: The Cape Eastern Line was built to serve the frontier, with its network of military forts, and strategically, the port of East London was chosen partly for being the closest port to the frontier for landing and transporting troops.
Split into simpler sentences: The Cape Eastern Line was built partially in order to serve the frontier, and its network of military forts. The port of East London was likewise chosen partly for strategic reasons, being the closest port to the frontier for landing and transporting troops.