Sahasram, it is

After a lot of wrangling, I've finally decided to base Sahadev and Asok in Sahasram. Other 'contending' towns were Rupanath and Kausambi.

Although Sahasram is neither on the Uttarapathas (main highways going Pataliputra to Takshashila) or Dakshinapatha (main highway Pataliputra to Supparaka port via Ujjaini), Sahasram was located on an alternate route to Ujjaini as well as on a highway that led to Southern provinces. The latter is more important because the southern route passes through Kalinga - read, Toshali - establishing the importance of Kalinga strategically placing a spy in Magadha, in a town that is easily accessible to both Pataliputra and the Kalinga intelligence, to which the spy from Sahasram will report.

Another thing is, the distance between Sahasram and Pataliputra is roughly about 90 miles, which would justify most scenes/activities of the story (royal hunting trips, Sahadev or Asok's trips to Pataliputra as physicians, Asok's escorting of Devi to Ujjaini). The good thing, also, about the alternate route to Ujjaini is that it is less frequently used, follows an 'uncertain' route across hills and thick forests west of Sahasram - this town in fact was a border-outpost town of the Magadhan kingdom - which makes it a perfect location for the events in the opening chapters of the book.

Kausambi came a close second, although I find it hard to justify why anybody would stop in Kausambi for too long after passing through more strategically important towns (Kasi, Prayag). The presence of these 'big' towns next to Kausambi dims its position, at least in my story.