Bolingbroke Castle in East Lindsey

You're standing at the ruins of Bolingbroke Castle, in the little village of Old Bolingbroke in Lincolnshire. There's not much left standing tall, but have a look around. You can see the low walls and the bases of the towers, tracing an irregular, many-sided shape across the grass. Now picture those walls up to five metres thick. And all around them, a water-filled moat more than thirty metres wide.