The scale and extent of monumental construction at El Pilar – over 150 hectares spread across two kilometers – attest to its position as a dominant Classic-period center in the upper Belize River area. Like several nearby sites in the Belize Valley, El Pilar was founded in the early Middle Preclassic as the focus of a burgeoning agricultural community. Major architectural expansion at El Pilar accompanied population growth across the area during the transition from Middle to Late Preclassic times. This paper explores the transformation of El Pilar from village ceremonial center to political capital at the onset of the Late Preclassic. New radiocarbon dates, stratigraphic reconstructions, and ceramic analyses reveal the massive scale of architectural investment in the ceremonial core of El Pilar – Plaza Copal – during this important transitional phase. We discuss these new data and their implications for understanding the increasingly complex sociopolitical landscape of the Late Preclassic upper Belize River area. The scale of this construction boom at El Pilar demonstrates the deep roots of power its ruling elites continued to wield until the close of the Classic period.