This chapter describes techniques of preservation of the main tower of the Bayon temple of Angkor Thom in Cambodia. The Japanese Government Team for Safeguarding Angkor was established in 1994 for the purpose of conservation work in the Angkor Archaeological Park. In this chapter, geotechnical and foundation studies of the main tower of the Bayon site are summarised with the emphasis on geotechnical engineering. The main tower of 31 m in height was constructed on a man-made sandy mound, 14 m in thickness, by direct foundation. Beneath the main tower, there is a vertical shaft in the foundation element that was backfilled with loose sand. The stability of the tower comes from the unsaturated, compacted soil mound. The character-defining elements are discussed in terms of the authenticity and stability of the foundation of the Bayon monuments.