The ancient name or the builder of this monastery known as Potgul vehera are not known.
But it has been suggested that it could be either the Kapila Vihara or Mandala Mandira built by King Parakramabahu the great (1153-1186 A.D.)
According to a stone inscription found at the site, it has been renovated by Queen Chandrawathie,a consort of King Parakramabahu the great .As its name implies if it is taken as a library shrine, this could be regarded as the oldest library complex so far found in the island.
The peculiar feature of this site is the circular brick building located in the square central terrace. Evidence suggests that the roof itself had been made of bricks and the inner walls contained paintings. On the four sides of the building four circular mounds are seen which may be the associate stupas, and on the lower terrace are seen square or rectangular residential cells.
(Description is extracted from Department of Archaeology) 
http://curhatz.blogspot.com/ : peninggalan sejarah nih … nice info
Hai…..