The site began as an early rath, a circular earthwork fort typical of early medieval Ireland, but was transformed around the late 12th century when the Anglo-Normans arrived. They converted this ancient Irish stronghold into a ringwork castle, essentially reshaping the existing earthen banks to create a defensive structure more suited to their military needs. According to historian Kieran O’Conor, the Fitzgeralds likely maintained this as a timber castle well into the early 14th century, making practical use of the site’s natural defensive advantages.

The year 1300 marked a turning point for Rathangan when the settlement was burnt, prompting John Fitzthomas to petition the King for assistance in rebuilding what he called a ‘fortalice’ there. Archaeological evidence suggests this wasn’t an entirely new construction but rather another modification of the existing ringwork castle. P.D. Sweetman’s research indicates that the original rath was altered once again during this period, reinforcing its role as a defensive stronghold. This rebuilt fortification appears in historical records as late as 1331, when it was referred to simply as ‘the castle’ in a contemporary survey of the area.

The medieval landscape of Rathangan was more complex than just this single fortified site, however. About 400 metres southwest of the ringwork castle stood a tower house, representing yet another phase of defensive architecture in the town’s evolution. This stone tower would have served as both a residence and stronghold for the local lords, complementing rather than replacing the older ringwork fortification. Together, these structures paint a picture of a strategically important location that was continuously occupied and fortified from the early medieval period through to the late Middle Ages, each generation of inhabitants adding their own defensive innovations to the landscape.