This dissertation reports on the results of field research at Chankillo, an archaeological site with 14 C dates ranging 400-100 Ca1BC in the lower Casma valley, Ancash region, Peru. The site is a large ceremonial center devoted to the cult of the Sun. It is known for containing one of the earliest solar observatories in the Americas. Another well-known component of the site is an impressive hilltop construction. When the research was designed, the prevailing view on the function of this building was as a setting for ritual battles. Thus, the main goal of the project was to evaluate whether it is a fortification, despite the persuasive claims to the contrary. If indeed a fortification, what does it protect? What can be learned about war and society from its study? To answer such questions, this dissertation presents the archaeological site context, architectural attributes, stratigraphy, artifact associations, chronology, function, and occupational history of the hilltop building. Its location, design, use, and abandonment are studied to reveal important dimensions of warfare, such as the scale, tactics, goals, target, intensity, and predictability of war, and the social scale and distance between fighting groups. Recognizing that such an extraordinary investment of time, labor, and materials in defense at the hilltop building reflects concern for the security of its inner structures, the excavations focused on reconstructing the function and history of the Temple of the Pillars within it. Its prominent, astronomically significant location and orientation, the presence of light-shadow casting devices atop stages for public performance for small groups with privileged access, and evidence of banqueting, indicate this temple was the center of a solar religious ideology.