General Willoughby continued to believe that the Japanese only intended to build airfields on the northern coast with the intent to attack Port Moresby and Australia by air. But the Japanese fought up the northern side of the Owen Stanley Mountains and in the middle of September, after weeks of fighting, descended the southern slopes to Ioribaiwa Ridge, within of Port Moresby. At the peak of their effort, the Japanese had 16,000 troops in the region. Japanese engineers remained on the coast to fortify the beachhead and build a system of reinforced and cross-linked bunkers.