This paper highlights the relevance of defensive architecture to archaeological discussions of monumentality, collective action, and political leadership. Recently, archaeologists have argued that some early religious monuments were built without permanent central leadership, raising important questions about the nature of collective action and the integration of large populations in communal tasks. We suggest that large-scale defensive architecture, when viewed as a form of monumentality, can contribute to these discussions. Evidence is presented from an archaeological investigation at Pucarani, a major hillfort settlement in the Lake Titicaca Basin of Peru dating to the Late Intermediate Period (or LIP, CE 1100–1450). In this time span after the collapse of the Tiwanaku state but before the rise of the Inca empire, the basin was dominated by large hillfort towns (Arkush, 2011). Pucarani is the single largest hillfort documented to date in Arkush’s multi-year investigations of 49 such sites in the northwestern Titicaca Basin, and its defensive walls are the largest by far. What kind of political leadership and social organization produced this impressive site? Our investigations of surface architecture and artifacts in the residential zone suggest social organization in several large corporate groups. Evidence for social differentiation and formal hierarchy is limited. Pucarani’s walls, though they were built in a short time frame and were quite visually imposing, required a labor input that was manageable for the large resident population. Defensive wall construction required planning and coordination, but was accomplished in heterogenous styles, possibly by multiple work groups. The evidence from Pucarani speaks to the capacity of LIP communities with loosely centralized leadership to organize and accomplish large-scale endeavors in the face of threat.