St Nicholas Cathedral began in the 13th century and still carries Gothic structure beneath later Renaissance and Baroque changes. Its layered interior is typical of Kalisz: not pristine, but full of evidence that the city has been rebuilt, repaired, and reworked for centuries.
The cathedral is close to the market square and pairs naturally with St Joseph's Sanctuary. Look for the older presbytery and the Polish Chapel, whose early 20th-century decoration adds another layer to the church's long life.