Nyköping is one of Sweden's oldest towns, founded in the 12th century on the estuary of the Nyköpingsån river where it meets the Baltic. It was briefly a royal seat — and the scene of one of medieval Scandinavia's most chilling betrayals: in 1317 King Birger invited his two brothers, Duke Erik and Duke Valdemar, to a Christmas feast at Nyköping Castle, then had them thrown into the dungeon and starved to death. The castle's round keep, Kungstornet, still stands as a reminder.
Today it is a quiet regional city of around 35,000, with a pleasant old town of wooden buildings and a waterfront that comes alive in summer. The Sörmland Museum occupies the castle buildings and covers the county's history from the Iron Age onward. The riverbank paths along the Nyköpingsån make for easy and enjoyable wandering between sights.
Nyköping is best known to many travellers simply as the location of Stockholm Skavsta Airport, 5 kilometres outside the city, used mainly by Ryanair as a southern Stockholm hub. That is a pity — the town deserves more than a transit stop. It is small enough to cover in a day but has a genuine, unhurried character that larger Swedish cities have lost. The Nils Oscar Brewery, one of Sweden's most respected craft producers, is based here and welcomes visitors. For those with more time, the Labro Meadows nature reserve on the eastern edge of the municipality is excellent for wildflowers and birds in late spring.