Many of the cottages here in Greenhead contain stones taken from the Wall. North of the village is Thirlwall castle. As part of its Scottish borders defensive work, it had a pele tower overlooking the bridge you crossed as you came into Greenhead. A pele tower had massively thick stone walls. The only entrance was by a double door at ground level. The bottom floor was a store room and those above were reached by a narrow curving stair. The upper floors were where people lived, and on the very top was a beacon for summoning help or warning of imminent attack.