 In 848 the archdiocese of Hamburg merged with the diocese of Bremen to become Hamburg-Bremen Archdiocese, with its seat in Bremen, and in the following centuries the archbishops of Hamburg-Bremen were the driving force behind the Christianisation of Northern Germany.In 888, at the behest of Archbishop Rimbert, Kaiser Arnulf of Carinthia, the Carolingian King of East Francia, granted Bremen the rights to hold its own markets, mint its own coins and make its own customs laws.The city's first stone walls were built in 1032.