In Mission Oriental, Kulang was a Sultanate at the extreme northeast of Malaya, bordering the South China Sea. Kulang was also the name of the main town, which contained, among other things, the Sultan's palace. Much of the country was hilly and covered with forest. What flat land there was had been cultivated as rice paddies which would be flooded or marshy after the rainy season. For these reasons, no airfield could be built in Kulang. The only suitable landing area for aircraft was a beach of firm sand a few miles long which was close to the palace. This beach had been used by the Japanese Army during the Second World War to land troop reinforcements.
At the time of the story, the jungles surrounding Kulang had been occupied by Communist terrorists, making overland travel out of the Sultanate unsafe, leaving only the option of evacuation by an aircraft using the beach.
Kulang is a fictional place. The Malaysian state of Kelantan is probably where Kulang is in the story, although Kelantan is not quite as hilly nor as isolated as Kulang.