The original castle on the site of current Château de Combourg was built around 1025 by Archbishop Guinguené, who gave it to his illegitimate brother Riwallon. Major alterations were made between the 15th and 19th centuries. The castle consists of four large, powerful buildings of dressed granite, with crenellations and machicolations, enclosing a rectangular courtyard. In each corner of this massive fortress is a round tower, also with crenellations and machicolations, with conical roofs. In 1761, the Chateaubriand family acquired the property and it was the childhood home of François-René de Chateaubriand (1768–1848).
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.