Angkor Thom was the royal city built by the Khmer Buddhist King Jayavarman VII, in the 12th century. Its name means "the big city." It has a square shape, 9 square kilometers, that you can explore on foot, but with the heat and the distances you'll be travelling to visit the temples, it can be a lot. It's most convenient if you only rent a bike for the day, worth 2 euros at most, and will be completely free. Also if you go with a group you can hire a cab with a driver that will take you around all day, but it's a bit more expensive, but avoids the cab drivers looking at you every time you want to change temples.