The Khmer Empire was profound for their great technologies including their smart water system.
Water System
The Khmer Empire's water system was a very advanced water system for their time. Sadly the water system was one of the reasons the Khmer Empire fell, but was still one of the most magnificent feats that the Khmer Empire had. The water system spread 1200 square kilometres and was one of the main reasons why the Khmer Empire flourished. When the drought happened it was disastrous because of all the water they had rely on was gone they ended up not being able to water crops and animals.
Building Techniques
Scientists don't exactly know what the Khmer Empire did to move great 300kg stones from a mountain quarry to the temples they were making. Scientist do have an liable answer on how they did this, The Khmer Empire must have cut out small rivers and used barrages to carry the stones to their destination, which would have worked great and would have been pretty effective.
Bronze Working
The Khmer Empires weaponry was very advanced as well and had a very good army. Most of the swords, hammers and other weapons that had to use metal such as knives, they used bronze instead of iron and steel because it was easier to cast also was more available for them at the time and was much stronger than iron.