Kastelle Priory was an Augustinian monastery founded in the medieval period at Konghelle — the Norse settlement that preceded Kungälv. The priory stood on a low hill about three kilometres south of the modern town centre. It was dissolved during the Lutheran Reformation in the 16th century and the buildings were subsequently dismantled, with stone taken to build other structures in the region. What survives are foundations and earthwork traces.
The site is not heavily visited and requires some effort to find, but it gives a tangible sense of the medieval landscape around Konghelle before the building of Bohus Fortress shifted the centre of power northward. A small information board explains the history. Access is on foot across farmland; the track from the road to the site takes about 15 minutes.