The site occupies a field known as Parc Castell and is believed to have functioned as the administrative and defensive centre for the commote of Maenorowen. Its construction likely dates to the period of Norman expansion into west Wales following the death of Rhys ap Tewdwr in 1093, a time when many new fortifications were established to consolidate Norman authority and control over strategically important valleys and coastal approaches.

Archaeological investigation suggests that the medieval castle was deliberately constructed within the remains of an earlier Iron Age hilltop enclosure. Evidence for this earlier phase includes a double bank and ditch along the better-preserved northern side, as well as a slight counterscarp bank following the steep, wooded eastern slope. These Iron Age features extend beyond the smaller medieval enclosure, indicating that Norman builders recognised and reused a site that had long been considered strategically advantageous. This layering of defensive works makes Castell Martin an important example of the long-term use of a prominent site for both protection and administration.

The medieval portion of the castle has survived largely due to its concealment within the trees of Old Castle Plantation. While much of the surrounding Iron Age enclosure was likely lost to centuries of agricultural activity, parts of the medieval earthworks remain discernible. The eastern bank sits atop a natural ridge overlooking a small river, providing both visibility over the surrounding landscape and control of the valley route. The northern corner of the Iron Age enclosure appears to have been reshaped into a D-shaped motte or partial ringwork measuring roughly 65 feet by 50 feet and rising about 12 feet above the surrounding ground, though tree growth has disturbed its summit.

Additional defensive banks are visible along the more vulnerable south-western side, while traces of the outer bank continue briefly into the adjoining field before fading under ploughing. The northern ditch is the best-preserved feature, approximately 10 feet wide and 4 feet deep, highlighting the original defensive intentions of the site.
The site name is also listed as Castell Fartin, translating to “Martin’s Castle,” is traditionally linked to the Norman knight Martin Turribus, a figure associated with the early consolidation of Norman power in Pembrokeshire.