THIS ARTICLE STUDIES THE FORT LA PLANCHADA, LOCATED IN SOUTHERN CHILE. CONSIDERING THE NATURE OF THIS NATIONAL MONUMENT, THE STUDY FOCUSES ON THE ANALYSIS AND CHARACTERIZATION OF ITS CONSTRUCTION ASPECTS: DESIGN, GEOMETRY, MATERIALITY AND CONSTRUCTION TECHNIQUES; ASPECTS THAT ARE PART OF THE POLIORCETICS (OR MILITARY ART) PERSPECTIVE. FROM A METHODOLOGICAL POINT OF VIEW, USING TECHNICAL HISTORICAL RESOURCES, RECONSTRUCTION OF THE FORT UNDER STUDY CULMINATED THROUGH A COMPREHENSIVE LITERATURE REVIEW AND ASSOCIATED FIELDWORK. THE ART OF POLIORCETICS AND CONSTRUCTION TECHNIQUES, AS USED BY MAJOR WRITERS OF THE 17TH CENTURY ABOUT FORTIFICATIONS AND THEIR PRINCIPLES, WERE APPLIED TO THE ORIGINAL CONSTRUCTION OF THE FORT, CONSIDERING THE DIFFICULTIES AND SOLUTIONS OF ITS TIME. THE CHARACTERISTICS OF THE ORIGINAL FORT AND THE ANALYSIS OF SUCCESSIVE REPARATIONS REVEALED THE EXISTENCE OF THREE DIFFERENT DESIGNS THROUGHOUT ITS HISTORY.