The headland where the Warta empties into the Oder was worth holding long before anyone thought of bastions. A settlement of Slavic Pomeranians stood here, on and off a tributary of Poland through the tenth century. Some three hundred years later the district passed to the Knights Templar, who were permitted to run a market under German law, and around 1261 Küstrin and the formerly Polish county of Lebus were absorbed into the Mark Brandenburg, which named its new eastern acquisition the Neumark. A fifteenth-century castle grew into something far more serious in the second half of the sixteenth, when the town served as the Neumark's capital and Italian-style bastion works were thrown around it. From 1535 to 1571 it was the residence of a junior branch of the Hohenzollerns. Seventeenth-century extensions made Küstrin one of the most heavily fortified towns in Germany, and in the 1880s a ring of four outlying forts — Czarnów, Gorgast, Sarbinowo and Żabice — pushed the defences further out.

A fortress on two rivers attracts sieges, and Küstrin collected them. In August 1758 a Russian army under Count Fermor was besieging the place when Frederick — the same Frederick, now king for eighteen years — came east to break the operation up, and lifted the siege by drawing the Russians off to Zorndorf in the marshes northeast of town. But the nineteenth century changed Küstrin more than any siege did. The railway arrived, industry followed quickly, and by the time Germany lost the First World War the place was manufacturing more than it was garrisoning. Most of the military installations came down after 1918. Then, under the Nazi government, Küstrin was rearmed and re-garrisoned, and the geography that had made it valuable to Brandenburg made it valuable again to a state that expected to be invaded from the east.

In January 1945 the Soviet advance reached the Oder and Küstrin was declared a fortress — a position to be held to the last, because the road to Berlin ran through it. The fighting lasted into March. When the town finally fell on 11 March 1945, close to ninety-five percent of its buildings had been destroyed, including all thirty-two of its factories. The fortifications that had survived Russians, French and two centuries of weather did not survive this. Neither did the streets of houses inside them, nor most of the ordinary lives that had been lived in those houses: the civilians who had not left were caught in a siege nobody was going to lift.

The postwar border followed the Oder, so the east bank went to Poland and the small western district of Kietz stayed on the German side as Küstrin-Kietz — one town cut in two, like Frankfurt an der Oder and Görlitz downstream and upstream. The German population was expelled. Then the rubble itself was put to work: the debris of the old town was loaded up and shipped to Warsaw, where it went into rebuilding a capital that German forces had systematically demolished in 1944. The castle where the young Frederick had been held was razed and its bricks went with the rest. Modern Kostrzyn nad Odrą was rebuilt inland, around the railway station east of the Warta's mouth, which is why the town of roughly 17,800 people has so little visible connection to the fortress that gave it its name.

Walk into the fortress today and you walk on cobbled streets with nothing along them. Foundations, kerbstones, cellar vaults, the outlines of houses in the grass — the plan of the old town survives in full while the town itself does not, and the effect is stranger than any ruin, because ruins at least stand up. The Berlin Gate is intact enough to walk through. Bastion Filip and the Chyżańska Gate hold their shape. Proposals to reconstruct parts of the old town surface periodically and then stall. What visitors actually come for is more mixed: German day-trippers taking the RB26 regional train from Berlin-Lichtenberg or Ostkreuz, a little over an hour to the end of the line, for cheaper shopping; birdwatchers heading east to Warta Mouth National Park; and, for sixteen summers between 2004 and 2019, several hundred thousand people a year arriving for one of the largest music festivals in Europe. Trains also run from Gorzów Wielkopolski, Szczecin and Zielona Góra.