The fortified town of Rello still maintains the original defensive walls and castle that were used as protection against civilisations that came from the south following the Douro River. It is the best-preserved walled enclosure in the province.
You can go inside through either one of the two gates located on the sides of the castle. The castle is protected by a fortified enclosure with circular and quadrangular towers that have elbow openings. The upper part of the towers are crowned with 15th-century machicolations and you can still see some remains of the keep, a water pool and a wall of the gate that separated the castle from the rest of the town.
In the outer enclosure, there are still gunboats in the lower section that were used for artillery.
References:Carlsten is a stone fortress built on the orders of King Carl X of Sweden following the Treaty of Roskilde, 1658 to protect the newly acquired province of Bohuslän from hostile attacks. The site of Marstrand was chosen because of its location and its access to an ice free port. Initially a square stone tower was constructed, but by 1680 it was reconstructed and replaced by a round shaped tower. Successive additions to the fortress were carried out, by the inmates sentenced to hard labour, until 1860 when it was reported finished. The fortress was decommissioned as a permanent defense installation in 1882, but remained in military use until the early 1990s.
The fortress was attacked and sieged twice falling into enemy hands. In 1677 it was conquered by Ulrik Frederick Gyldenløve, the Danish military commander in Norway and in 1719 by the Norwegian Vice-Admiral Tordenskjold. At both occasions the fortress was returned to Swedish control through negotiations and treaties.