In February 1534 the catholic Bishop Franz von Waldeck provoked by the establishment of an Anabaptist enclave in the North Rhine Westphalian city of Münster sought to regain control of his diocese laying siege on the city. For the first time, thanks to eyewitness accounts, a Mighty Fortress provides an insight into the stages of the siege detailing the failed assaults, the erection of a ring of earthworks and finally the act of treachery which caused the collapse of a sixteen-month defence by the Anabaptists.