The Santa Clara buildings were originally built by King Alfonso XI as his palace in 1344. His son Peter the Cruel had it embellished by Mudéjar artists, beautiful works at Santa Clara, though on a much smaller scale than they did in the Alcázar of Seville. The facade, a lovely small patio, a chapel and the baths remain of Peter the Cruel's palace. Blanche de Bourbon was held here after her abandonment by Peter for María de Padilla in 1353. The former portal, blocked off now, has a particularly fine Mudéjar doorway. In 1363 he ceded Santa Clara to two of his daughters by María de Padilla. They turned it into a convent, but it retained its role as a royal palace.
Santa Clara convent's saddest association is with Joanna I Queen of Castile and Aragon, the daughter of Isabella I of Castile and Ferdinand II of Aragon, widow of Philip I of Castile and mother of his six children including Charles V, Holy Roman Emperor - popularly known as Juana la loca (Joanna the mad) - who was confined here for almost half a century until her death in 1555.
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.