Structural Depth Theory I: Foundations of Structural Constraint Geometry via Grothendieck Sites This work introduces the mathematical foundations of Structural Depth Theory, a research program investigating structural consistency, local-to-global organization, and constraint-induced admissibility through the formalism of Grothendieck sites and sheaf theory. The paper develops a minimal categorical framework based on Structural Constraint Sites, avoiding assumptions of predefined state spaces, dynamics, metrics, or probabilistic structures. Instead, admissibility is derived from covering and gluing conditions encoded by a Grothendieck topology. The work serves as the opening contribution of the Structural Depth Theory program and establishes the mathematical groundwork for future studies on structural diversity, constraint density, information organization, and local-global consistency. Related research materials, publications, and ongoing developments are available at: marketalchemy.io Keywords: Structural Depth Theory, Grothendieck Site, Sheaf Theory, Topos Theory, Category Theory, Structural Geometry, Constraint Geometry, Local-to-Global Consistency, Information Organization, Mathematical Foundations.