 Castle Greg was a most likely used as a monitoring base for an east-west road running along the foot of the nearby Pentlands, from the Forth to the Clyde Valley.Although the fortlet currently commands no long-distance views, during the 1st century AD, when the fort was in use, the surrounding countryside was not forested, and Castle Greg would have been able to view clearly up to the Fife coastline and the mountains beyond.The name Castle Greg is possibly derived from the Roman name "Camulosessa Præsidium", from nearby Camilty, itself derived from "Camulos Tref" – literally, village of Camulos.