Along the Cumbrian coast between Silloth and Maryport, the Roman frontier did not end when Hadrian's stone Wall reached Bowness. A chain of milefortlets and towers continued down the shore, watching the Solway for raiders arriving by boat, and Tower 20b near Swarthy Hill is one of them. It was a small stone watchtower, probably manned by a handful of soldiers, and what remains is a low outline in the turf above the beach. You come for the context and the view across the water rather than for the masonry.
That view is a fine one. Looking north-west across the firth you see the rounded bulk of Criffel above the Scottish shore, and on clear days the hills of Galloway stretch away beyond it. The beach below is a mix of shingle, sand and rock scars, with dunes and coastal grassland held together by marram and sea holly. Natterjack toads breed in a few spots along this coast, and the evening light out over the Irish Sea, with the Isle of Man sometimes visible, is reason enough to linger.
The B5300 runs along this coast from Silloth through Allonby to Maryport, and it is an easy, enjoyable road: mostly straight, open to the sea on one side, with gentle bends, a few village speed limits and wide verges where people park to walk the beach. Wind is the main hazard, often strong and gusty off the sea, and sand blows onto the road in places. On a sunny weekend Allonby fills up with day trippers and ice-cream queues, but on a weekday evening it can be almost empty.
Maryport, five kilometres south, is the place for fuel and food, and it repays an hour. The Romans had a substantial fort there called Alauna, and the Senhouse Roman Museum on the cliff above the harbour holds a remarkable collection of altars dedicated by commanders of the garrison, dug up from pits near the fort. The town itself grew in the eighteenth century as a coal and shipbuilding port and still has a handsome harbour. Allonby, closer to the tower, has a café or two and a few benches that face exactly the right way.