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:Bomarsund fortress and garrison was built in 1832-1854 by Russian Empire to defence Åland Islands against enemies. British and French fleet attacked against it during the Crimean War in 1854. After a week of fighting the British stormed the remaining fort. Anglo-French forces destroyed it totally after battle. In the Treaty of Paris 1856, the entire Åland Islands were demilitarized, which is a status that has been preserved until this day.
Nowadays there are some ruins left and fortress area is open for visitors.