Note: "In around 1285, the Count of Holland Floris V (1254 – 1296) ordered the construction of a stone keep at a strategic location at the mouth of the river Vecht, the most important waterway to the diocese of Utrecht. In 1296, Floris V was kidnapped during a falcon hunt and later murdered by his own nobles and his castle was destroyed. From 1370, the original Muiderslot was restored and expanded with a residential wing facing the Zuiderzee. The castle was strengthened with earthen walls and bastions in 1576." Source: The Muiderslot official website (English translation available)
The original Muiderslot has a dark and violent history. Floris V apparently built the castle in order to tax, i.e., shakedown, all the trade passing through the region on the River Vecht. "In 1296," according to Castles of Medieval Times, "he was imprisoned in his own castle by Geeraerdt van Velsen, because Floris V had ordered the execution of Van Velsen's brother on false charges. Floris V managed to escape from the castle but was killed while fleeing."
The English sources I've found are sketchy and contradictory, but the real story of Muiderslot seems to be a lot more convoluted and bloody than suggested above. According to Wikipedia, for instance, van Velsen killed Floris because he raped van Velsen's wife, who then committed suicide. It also sounds like van Velsen was involved in a plot to overthrow the king. The sequence of events is obscure, but at some point the king apparently executed van Velsen's brother for treason. Whatever happened, van Velsen had Floris imprisoned, but the king managed to escape -- temporarily. Van Velsen is said to have stabbed the king to death personally, only to be captured later, tortured for a couple of days, then hanged, drawn and quartered.
And so it goes: A fairly typical example of royal mayhem.
Medieval kings had more in common with Tony Soprano than with the monarchs of chivalric legend. And their nobles weren't any different. Their "authority" was based on physical violence, so it's not surprising that their lives were chronicles of blood, betrayal, intrigue and vicious crimes. The violent careers of statesmen is a natural consequence of the fact that the State is the monopoly on violence over a given territory. Nothing has really changed since then, but ancient and medieval history gives us a look at the State in its most basic form, undiluted by the myths and self-serving propaganda of modern political machines. History is made by killers. That's the bottom line.