Nine metres below Staroměstské náměstí, a network of stone cellars and passages extends across three hectares of medieval Prague, preserving layers of the city's foundations dating to the twelfth century. The underground labyrinth exists because the Old Town repeatedly raised its street level to combat flooding from the Vltava, burying entire ground floors beneath centuries of sedimentation and reconstruction. What shopkeepers and merchants once used as storage vaults, workshops, and living quarters now form one of Central Europe's most intact subterranean historical sites, accessible only through guided descents from Romanesque entry points. The cellars beneath Old Town Hall and the surrounding blocks reveal architectural strata spanning Gothic, Renaissance, and Baroque periods. Exposed foundation walls show chisel marks from thirteenth-century masons. Barrel-vaulted chambers retain original stone flooring, wells, and drainage channels carved when the streets above were two storeys lower. Historians have identified wine cellars, tannery pits, and grain storage rooms, their purposes legible from residual fixtures and structural adaptations. The underground preserves a vertical cross-section of urban life, each layer corresponding to a documented phase of Prague's expansion and fortification. A Prague medieval underground guided tour navigates this multi-period complex, interpreting architectural features and spatial relationships that surface-level ruins cannot convey. Guides illuminate how citizens adapted subterranean spaces during sieges, plagues, and economic shifts, using the cellars as refuges, smuggling routes, and centres of craft production. The tours link underground findings to documented events—Hussite conflicts, Habsburg rule, and nineteenth-century modernisation—providing material evidence for historical narratives often confined to archives. The experience requires controlled access and expert commentary; the passages are not self-explanatory, and their preservation depends on regulated visitor flow. The site's significance extends beyond tourism. Archaeologists continue to study ceramic fragments, construction techniques, and organic residues embedded in the stone, refining chronologies of medieval urban development. Conservation teams monitor humidity and structural integrity, balancing public access with the fragility of centuries-old masonry. The underground functions as both monument and active research field, offering a tangible connection to the medieval citizens who shaped Prague's commercial and social infrastructure beneath the visible cityscape.