The Role of Muenster
The Holy Roman Empire and Sweden declared the preparations of Cologneand the Treaty of Hamburg to be preliminaries of an overall peace agreement. This larger agreement was to be negotiated in Westphalia, in the neighbouring cities of Muenster and Osnabrueck. Both cities were to be maintained as neutral and demilitarized zones for the negotiations. Münster was, since its re-Catholization in 1535, a strictly mono-denominational community. It housed the Chapter of the Prince-Bishopric of Muenster. Only Roman Catholic worship was permitted. No places of worship were provided for Calvinists and Lutherans.
Both cities strove for more autonomy, aspiring to become Free Imperial Cities, so they welcomed the neutrality imposed by the peace negotiations, and the prohibition of all political influence by the warring parties including their overlords, the prince-bishops.
Since Lutheran Sweden preferred Osnabrueck as a conference venue, its peace negotiations with the Empire, including the allies of both sides, took place in Osnabrueck. The Empire and its opponent France, including the allies of each, as well as the Republic of the Seven United Netherlands and its opponent Spain (and their respective allies) negotiated in Muenster. | Source: http://en.wikipedia.org/wiki/Peace_of_Westphalia, June 18, 2011