A comparison of two different mechanical separation procedures, a settling process and sieving process, was performed in the samples preparation for radiocarbon dating of archaeological lime mortars from the Tower Keep at Irulegi Castle (Navarre, Spain). The different fractions obtained by the two different mechanical separation procedures were converted to carbon dioxide by sequential dissolution in order to compare the yielded 14C ages. Fifteen AMS 14C measurements were performed from the lime mortars, as well as for a tooth and a charcoal fragment embedded in the mortar. Samples obtained by the settling process (grain-size window of 0.5–2 μm) led to more accurate radiocarbon results than samples obtained by sieving (grain-size window of 46–75 μm). The results point to Irulegi Castle existing during the 10th century as well as to the renovation of the castle defences and different repairs in the Tower Keep during the 13th and 14th centuries. Mineralogical characterization of samples was essential when discussing the radiocarbon dating results. For this end, petrographic microscopy, X-ray diffraction, thermogravimetric analyses and cathodoluminescence analyses were performed.