The first documents testifying to the existence of a castle in Castro date back to the 13th century: a document of 1282 described it as having a strategic importance to protect the kingdom. It is quite sure that the castle was built on the ruins of a former Byzantine building, which protected the fortified Roman village. In 1480 it was destroyed and then rebuilt and strengthened  in 1572 by the Spanish viceroy.
On the north-western part, stand the thick walls of the Bastion of the buttress, with a single central room and fortified battlements. The Torre Cavaliera, the highest and most imposing one, is a three-story tower with three rooms, covered with barrel vault. The Torre Circolare, dating back to the Aragonese period, has three rooms with small windows and merlons with machicolation.
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The Castle of Gruyères is one of the most famous in Switzerland. It was built between 1270 and 1282, following the typical square plan of the fortifications in Savoy. It was the property of the Counts of Gruyères until the bankruptcy of the Count Michel in 1554. His creditors the cantons of Fribourg and Bern shared his earldom. From 1555 to 1798 the castle became residence to the bailiffs and then to the prefects sent by Fribourg.
In 1849 the castle was sold to the Bovy and Balland families, who used the castle as their summer residency and restored it. The castle was then bought back by the canton of Fribourg in 1938, made into a museum and opened to the public. Since 1993, a foundation ensures the conservation as well as the highlighting of the building and the art collection.
The castle is the home of three capes of the Order of the Golden Fleece. They were part of the war booty captured by the Swiss Confederates (which included troops from Gruyères) at the Battle of Morat against Charles the Bold, Duke of Burgundy in 1476. As Charles the Bold was celebrating the anniversary of his father's death, one of the capes is a black velvet sacerdotal vestment with Philip the Good's emblem sewn into it.
A collection of landscapes by 19th century artists Jean-Baptiste-Camille Corot, Barthélemy Menn and others are on display in the castle.