Chan Chan, the prehistoric capital of the Chimu culture (ca. A.D. 900 to 1450), is located in the Moche Valley close to the Pacific Ocean on the North Coast of Peru. Its sandy desert environment is dominated by the dry onshore turbulent and gusty winds from the south. The nucleus of this large urban community built of adobe is visually and spatially dominated by 10 monumental rectilinear high walled citadels that were thought to be the domain of the rulers. The form and function of these immense citadels has been an enigma for scholars since their discovery by the Spanish ca. 1535. Previous efforts to explain the citadels and the walls have emphasized the social, political, and economic needs of the culture. The use of the citadels to control the effects of the wind, sand, and dust in the valley had not been previously considered. Through the use of theoretical constructions and wind tunnel experiments, it is established that the form of the classic variant of the citadel was developed from a longtime interaction between the man—built environment and the natural environment. The Chimu had designed a courtyard system that reduced stress and discomfort from wind, sand, and dust by means of architectural features that included: the rectilinear citadel plan with the long axis parallel to the prevailing winds; the contiguous courtyards with the long axis in common; the high exterior walls; the high interior transverse walls; and the triangular cross section of the walls. It is demonstrated that these features kept out the blowing sand, reduced the wind speeds at pedestrian level, and kept dust, entrained in the airstream by the anthropogenic activity outside the walls, from entering the enclosures. It is also demonstrated that there is a correlation between the degree of protection afforded in a sector of the citadel and the social, political, and economic activities that took place in that sector. / PH. D.