The castle on Kozlov Rob above Tolmin is one of the most important historical sites in western Slovenia. Over the years, large quantities of finds dating from the late medieval and early modern periods have been recovered from the castle ruins in the process of archaeological investigations and construction works. Many of the recovered iron artefacts are extremely difficult to identify due to their fragmentary state. This paper presents the study of four relatively recently discovered artefacts. The available evidence suggests that at least one of them belongs to a helmet, while another has been identified as the remains of a lock. The artefacts were analysed with X-ray radiography and X-ray diffraction. Additionally, samples were taken for microhardness Vickers measurements and metallographic analysis with a light microscope and a scanning electron microscope. Additional microchemical and structural phase analysis were done with the electron microscope. The aim of archaeometallurgical analyses is to contribute to a more precise interpretation and add a new dimension to our knowledge of the castle site at Kozlov Rob.