 Most parts of the hilly ground are covered by jungle, but most of the trees shed their leaves seasonally.The city of Koh Ker was on the most important strategic route of the Khmer empire.Coming from Angkor and Beng Mealea to Koh Ker this road led to Prasat Preah Vihear and from there to Phimai in Thailand and Wat Phu in Laos .