Puranic Background:

According to legends, it is believed that, Vibhishana the brother of demon king Ravana of Swarnalanka requested Lord Rama to grant him the permission to carry the Archamurthy (idol of Sri Ranganatha Swamy) to his home town for worship. However, he could not carry it beyond the place near present-day Thiruchirappalli. Vibhishana obtained the promise from Lord Ranganatha Swamy that he would always be watching over his land from the place of installation.