Having a lot in common with Angkor Wat, Phimai shows a great example of classical Khmer architecture. Ancient Khmer architects were best known for their superior use of sandstone over the traditional bricks and laterite architectures. Sandstones are used on the outer layer where they are visible. Laterite on the other hand was used for the outer wall and other hidden parts. All the structures are huge sandstone blocks. There are many lotus shaped roofs which represent Mount Meru (A holy mountain in Hinduism).
These Khmer temples, Phimai in this case, were intended to resemble the universe. The main building resembles the peak of the mountain that centers the universe. The surrounding walls resemble the water and encircling mountains. The Khmer didn’t develop the technique of true Vault architecture during their time. This results in that large areas could not be roofed over. They didn’t have the motivation to come up with how to develop the technique of true vaulting because their religion didn’t require this. For them, large open areas are fine. They instead developed the use multiple chapels.