Polonnaruwa
The medieval capital: reservoirs, council chambers and a long archaeological park.
Polonnaruwa rewards a full day and a bicycle or a car inside the site, not a photograph of the Gal Vihara on the way to somewhere else. The scale is the point: palaces, rank of image houses, and the tanks that still hold water.
We put it in the Cultural Triangle classics as its own day, with the night still in the Sigiriya–Dambulla area. Anuradhapura is the older city, further north, and we add it only when you have another night to give it.