Park on the D5 at the marked parking space near the kilometer post number 30. This is not far south of the junction of the D5 with the D2 at Thorenc. Your route to the cache heads up the mountain along an old Roman way.
Castellaras is the ruins of a fortified place (oppidum) perched at 1,400m - a chapel, a small palace and a few houses, all surrounded by the village walls. It's a fascinating place to explore; but even better, bring a picnic and eat it looking out over the amazing views.
Note on cache maintenance: I'm now living in Vence, which is about 45 km away from this cache.