Berwick Castle is a ruin, and not a very complete one — the railway station, built in 1850, destroyed much of it. What survives is the White Wall, the western curtain facing the river, a long stretch of thirteenth-century stonework that gives a good impression of how formidable the original fortification must have been. The castle controlled the bridge crossing and with it the main route between England and Scotland.
Access to the visible remains is free, and the riverside walk below the castle walls gives the best views. The castle's history is intertwined with the most violent episodes of the Anglo-Scottish Wars — it changed hands repeatedly between 1296 and 1482, roughly corresponding to Berwick's thirteen recorded changes of national sovereignty. The nearby tourist information centre has good material on the full history.