Additive manufacturing (AM) has revolutionized the fabrication of complex structures and is increasingly combined with topology optimization to create lightweight designs with minimal material usage and maximum stiffness. This study explores the design of lightweight structures intended for casting in AM-produced formwork, a problem stemming from the building industry. Unlike previous research that directly applies topology optimization to AM-fabricated structures [1,2], this work focuses on temporary formwork, which serves as a mold for casting rather than being part of the final structure. A key challenge in this process is that the formwork must remain free of internal supports in the casting region, as such supports would obstruct the flow of casting material. Moreover, topologies featuring internal cavities are incompatible with casting, as they can lead to disconnected or floating parts. To address these challenges, this paper proposes a novel topology optimization framework specifically tailored for designing castable structures with AM-fabricated formwork. The approach simultaneously ensures: (i) the formwork is self-supporting during printing, thereby eliminating the need for additional supports, and (ii) the optimized structure avoids internal cavities to guarantee continuous connectivity. The method employs a five-field density representation combined with an AM formwork filter to enforce manufacturability constraints. Additionally, the framework allows for customized printing directions, enabling the generation of diverse topological structures and formworks. The effectiveness of the method is demonstrated through four numerical examples: cantilever beam, MBB beam, slab and wall, followed by an experimental validation to assess practical feasibility. The results confirm that the formwork can be printed without extra supports and that internal cavities are successfully eliminated. This work offers a new strategy for fabricating lightweight structures alongside their compatible structural formwork, contributing to enhanced efficiency in construction and structural engineering.