As mentioned before, Polonnaruwa is a top whs in Sri Lanka. I visited the city on a 3 day tour to the cultural triangle that also included Dambulla, Sigiriya and Anuradhapura. A round ticket is a must, as it covers all sites (except Dambulla) for 50 USD. Single entry would be 25 USD for each site (foreigners pay about 20 times more than locals for most sites in Sri Lanka). Don't miss the quadrangle with its beautiful buildings. The site is extensive and there is a lot to see. You need private transport to cover the whole site in one day.