In the 11th century, there already was here a fortress called castellum montis admirabilis, "castle with a nice view". Until the 15th century, the castle belonged to the Gouët family. The current castle was raised by Charles IV d'Anjou, whose family kept it until 1659.
Then, here was the prince of Conti: he married the legitimate daughter of Louis XIV and Louise de La Vallière, Mrs de Blois, and gave Montmirail to his widow in 1685. In 1719, she sold it to the marquis de Neuilly, then to the Guillebon, the Boisguibert and the Fayet.
Did you notice the austere façade, overlooking the street? The Classical façade with a view on the garden, with the pediment flanked by Fayet's family blazon, is more elegant! Here, in this castle, Guillaume IV Gouët invited the king of France Louis VII and king of England Henri II Plantagenet. Guillaume was king of England Henri I's grandson, so maybe that's why they chose Montmirail as a place for the meeting gathering Henri II and Thomas Beckett, archbishop of Canterbury?
Thomas conflicted with Henri II about Church's privileges. So he was exiled, and moved in France in 1164. His king wanted to arrest him and finally king of France asked for a reconciliation, on January 6th and 7th 1169, in Montmirail castle. We know how's the story end: the two men started to fight again. Thomas was murdered on December 29th 1170, in the cathedral of Canterbury...