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A geophysical survey (magnetic gradiometry, magnetic susceptibility and ground resistivity) was undertaken of the interior of the hillfort, together with an area outside the hillfort entrance to the east. The interior of the hillfort proved to have been very heavily disturbed by deep ploughing during land "improvements" in the mid-twentieth century, and few distinct ancient features could be discerned, although areas of activity were suggested in a zone immediately inside the ramparts on the northwestern and northern sides. No ancient features were identified outside the fort.
The area between the two ramparts in the area of the entrance (near the southeast angle of the hillfort), however, showed significant magnetic anomalies, including features close to the inner face of the outer rampart which were strongly suggestive of metalworking hearths or furnaces, in an area in which small slag particles were recovered from the surface. A small collection of larger slag pieces from the site, probably collected over twenty years ago, was located in the collections of the Archaeology Section, Cardiff University. These pieces were not closely provenanced, but taken together with the small slag particles retrieved during the geophysical survey, form reasonably strong, although not conclusive, evidence for iron smelting in a non-slag tapping furnace. Evidence for such furnaces has not yet been found in South Wales, although they are becoming well known from North Wales and various parts of England, where they appear as the dominant furnace type in the pre-Roman Iron Age. If this identification can be confirmed by further work, it would give the Caerau Hillfort great regional significance. Iron oxide ores crop out around Mwyndy, 1500m south of the hillfort.