Bonilla de la Sierra castle has three wall sections and three turrets of different styles and origins. The main par of the castle was formed by the Tower of Homage, of the 14th century, it has matacanes and round arches, and under the tower is the main yard and the rooms. The castle contains 14th-century paintings.
It was the resting place of the prelates of the Ávila bishopric, and held important episcopal synods, as the one in 1384. Bishop Sancho Dávila had a wall built around the village in the 14th century.
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.