Archaeological evidence indicates that Caer Seion was occupied at least as early as the 6th century BC. Like many iron age communities in Wales, it is located at the top of a hill, in this case Conwy Mountain, with a magnificent view of the surrounding countryside, including the mouth of the Conwy River.

Caer Seion is unusual in that the main fort contains a smaller, more heavily defended fort, complete with its own distinct defenses and entrance, but with no obvious means of access between the two. Speculation has suggested that it was accessed by ladder or perhaps even a high walkway, like a battlement. The second fort appears to have been occupied starting around the 4th century BC. It does appear that both forts were occupied simultaneously for roughly two hundred years, until the 2nd century.

The larger fort contains upwards of fifty round houses, and the inner fort only six. The total area is around 7.5 acres and roughly 1000 feet by 300 ft. The larger fort was protected by a single wall on three sides, leaving a fourth side defended only by the natural shape of the mountain. In the hut closest to the entrance of the fort, investigators found over six hundred sling stones, indicating the hut acted as a gatehouse and that the occupants of the fort had reason to prepare to defend themselves.

Other finds at the site include a possible pair of iron tweezers, a small fragment of corroded iron that might be a lancehead or knife, a number of stone spindlewhorls, four saddle querns, whetstones, pot boilers, and bone fragments identifiable as from oxen, horses and sheep.