There he led the excavations of Izborsk (1924) and Grobin in Latvia (1929), among other early medieval sites.
The Norsemen may have remained in control of Grobin until the mid-9th century, when — as Rimbert's Vita Ansgari relates — people inhabiting the Courland regions of Latvia rebelled after a long period as tributaries of Swedish rulers.
Following the decline of Truso to the south and Grobin to the north in the course of the century, Kaup succeeded them as the principal regional colony of Swedish merchants from Birka.