This study explores the “Tanka”, a traditional underground rainwater harvesting system situated within the historic Jaigarh Fort, on the outskirts of Jaipur, Rajasthan, India. The research focuses on understanding the functional role and engineering benefits of organic and mineral admixtures historically used in its construction, alongside insights into production techniques and layering mechanisms. Detailed analysis revealed the significant contribution of pozzolanic admixtures in enhancing hydraulic reactions, improving water resistance, and increasing durability within lime-based plaster systems. Additionally, the incorporation of organic fibers, such as straw, was identified in the plaster, demonstrating improved crack resistance. The results highlight the synergy between these traditional admixtures and local materials, refining porosity while contributing to long-term structural stability and performance. Furthermore, the study examines the vernacular production technology, including the sequential layering of plaster and the influence of aggregate size. The layering method facilitated optimized adhesion and resilience, while aggregate size impacted the permeability of the structure.