According to my guide at Polonnaruwa, the area once played host to different schools of Buddhism. A notable byproduct of this is the different styles of the Stupas that were built and can still be seen. These two pictures illustrate the Theravada style (top) which is now the dominant school in Sri Lanka, and the Mahayana school (bottom), of building Stupa’s. As you can see, the Theravada style is more rounded (and is in better condition), while the Mahayana style is stepped.