The Pennine Way has brought you at last to the Roman Wall. Built in the 2nd century AD, it originally ran for 73 miles from the Solway Firth to the Tyne. It was 6 metres high and up to 3 metres thick. You are going to walk along it as far as Housesteads, the spectacular site of the remains of a fort, an ideal place for you to brief yourself on the wonders of the Wall. 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 that 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 staircase. The upper floors were where people lived, and on the very top was a beacon for summoning help or warning of imminent attack.