Former settlements, now abandoned, are found in inland upland locations on many larger islands in the tropical Pacific. In Fiji, such settlements are known today as koronivalu (war-towns) and, as elsewhere in the region, appear to have been established within the same period during the first half of the last millennium. Twenty-seven koronivalu were mapped for this research in the Bā Valley and nearby Vatia Peninsula, northern Viti Levu Island (Fiji); of these, nine were subject to detailed investigation. All koronivalu are in defensible locations, either with exceptional views across the surrounding landscape or hidden within deep narrow valleys. At all koronivalu, evidence for the consumption of marine shellfish was found, even though the sites are often far from the coast. Twenty-four radiocarbon ages from charcoal and shellfish remains were obtained. A single age around a.d. 700 from the farthest inland site (Koroikewa) appears anomalous. The remainder, once adjusted, suggest that most koronivalu in the study area were established a.d. 1200–1750, perhaps separable into early (a.d. 1200–1450) and later (a.d. 1500–1750) phases. While questions remain about the functions of these koronivalu, the fact that, as elsewhere in Fiji and in other western Pacific Island groups, they appear to have been established within the same period suggests that there is a region-wide explanation for the profound settlement-pattern change this implies. Climate change, perhaps expressed through drought and/or sea-level change, appears the only plausible external forcing mechanism.