This study characterises the mortar materials used in the construction of walls and floors at the Arroyo de la Dehesa de Velasco site, located near the Roman city of Uxama Argaela (the modern Burgo de Osma—Ciudad de Osma, Soria, Spain). Multilayer mortars have been characterised by petrographic, mineralogical (X-ray diffraction and scanning electron microscopy with energy dispersive analyses and geochemical analysis (X-ray fluorescence). Additionally, radiocarbon dating of the mortar binder fraction was performed in order to establish the chronology of the building in the absence of other archaeological chronological records. The results showed that similar siliceous aggregates and lime binders were used in the fabrication of multilayer system mortars. Some multilayer wall mortars show ceramic fragments or brick powder to produce hydraulic mortars and improve the resistance to moisture. The raw materials used for the construction of the site were of local origin and the construction was built during the first century BC, according to radiocarbon dating.