Weissenau Castle was first mentioned in the historic record as castrum Wissenowe in 1298. It is unclear whether it was built by the Lords of Rotenfluh (Unspunnen) and then given in fief to the Freiherr of Weissenau or if it was built by the Weissenau family as the center of their estates. The castle was built on what was an island at the mouth of the Aare river into Lake Thun. In the intervening centuries, the waterway silted up and the island became connected to shore. The market town of Widen grew up across the channel from the castle and in 1362 was connected to the castle by a bridge. Around 1334, the Freiherr of Weissenau joined other local nobles in a war against the growing power of the city of Bern. After the defeat of the nobles, Weissenau was forced to sell the castle and Widen to Interlaken Abbey to pay his debts. In 1365, the Abbey moved the weekly markets and yearly fair away from Widen and to the village of Aarmühle (which is now Interlaken). Losing the market devastated Widen and it began to decline.
In 1528, the city of Bern adopted the new faith of the Protestant Reformation and began imposing it on the Bernese Oberland. The Abbey and its villages joined in an unsuccessful rebellion against the new faith. After Bern imposed its will on the Oberland, they secularized the Abbey and annexed all the Abbey lands. Weissenau Castle and the small village of Widen became a part of the Bernese bailiwick of Interlaken. The castle was used as a prison into the 16th century, but began to fall into disrepair and eventually collapsed. In 1655 and again in 1700 the bailiwick made plans to renovate and repair the castle. However, neither plan was implemented.
Today only ruins remain of the castle. The castle residence and tower, portions of the castle building and the curtain wall are all still standing.
References:Saint-Georges de Boscherville Abbey is a former Benedictine abbey. It was founded in about 1113 by Guillaume de Tancarville on the site of an earlier establishment of secular canons and settled by monks from the Abbey of Saint-Evroul. The abbey church made of Caumont stone was erected from 1113 to 1140. The Norman builders aimed to have very well-lit naves and they did this by means of tall, large windows, initially made possible by a wooden ceiling, which prevented uplift, although this was replaced by a Gothic vault in the 13th century. The chapter room was built after the abbey church and dates from the last quarter of the 12th century.
The arrival of the Maurist monks in 1659, after the disasters of the Wars of Religion, helped to get the abbey back on a firmer spiritual, architectural and economic footing. They erected a large monastic building one wing of which fitted tightly around the chapter house (which was otherwise left as it was).