The monastic buildings at the foot of the Mihintale Hills, just beside the begin of the stairway, are a small Arama, "Arama" being the Pali word for Sanskrit "Ashrama". It's another word for monastery. But in the case of Mihintale and other large Viharas, it's just a term for a separate group of buildings. The group of five ancient buildings, named "Mani Naga Mandira", is separated from the surrounfings by an enclosure wall called Prakara in Southasian art. It's demarkating a sacred area.  In this case, the Arama compound is a square if 40 m length. 

The Arama consists of a group of five square buildings. They are called Prasadas.Usually this term means "palace". But it's also a word for pavilions, as ancient unfortified palaces often were groups of pavilions in a garden. Only pillars and foundations survived, as they are constructed of stone

From www.lanka-excursions-holidays.com