Milefortlet 21 lies to the south of Allonby on the Solway estuary. It is part of the extension to Hadrian’s Wall that runs down the coast of Cumbria to the fort at Maryport. It stands towards the summit of Swarthy Hill looking across the mouth of the estuary.
The Roman fort at Maryport is in direct sight of the milefortlet (on the horizon, to the south). The fortlet itself was excavated in 1990 and 1991, and has since been laid out for visitors.
The fortlet provides good observations across the Solway to Galloway. Note the later saltpans on the edge of the shore.