Concrete core samples extracted from the base of the 50-year-old reactor building at the Unterweser nuclear power plant in Germany were analyzed using contemporary and complementary experimental techniques. The analysis focused on qualitative and quantitative assessments of the mineralogical composition of ingredients, mixture composition, aggregate size distribution, presence of cracks, porosity, and compressive strength. Optical microscopy revealed insignificant micro-cracking in the concrete matrix, while the presence of blast furnace slag was clearly visible. Petrographic analysis of polished and thin sections has enabled the estimation of the concrete mixture composition and the grain size characteristics of the aggregates. The aggregates consist of quartz, silt, and volcanic rock with minor amounts of fossil-bearing grains. Pore size distribution obtained from mercury intrusion porosimetry (MIP) reveals that the concrete has a relatively refined pore structure. Hydration products in the matrix, i.e., hardened paste, were analyzed using powder X-ray diffraction (p-XRD) and thermogravimetry (TGA). Furthermore, backscattered electron (BSE SEM) image analysis was conducted to quantify the original mixture composition, including cement and slag contents, and water/binder ratio. This study emphasizes strategic and practical aspects of conducting in-depth analysis on hardened concrete samples that are several decades old and may serve as a general guideline for similar inquiries in diverse concrete structures.