 Wallenstein and Tilly subsequently gained central and northern Germany, joined their forces during the summer of 1627, invaded Danish Holstein in September, and advanced through the Jutland peninsula as far as Limfjord.While Christian IV's control of the western Baltic Sea and the Danish isles was unchallenged, Wallenstein was given the Duchy of Mecklenburg "vis-a-vis" Denmark in January 1628, and was preparing to construct a navy of his own at the occupied ports of Ålborg, Wismar, Rostock and Greifswald starting in late 1627.Christian responded with an amphibious campaign in 1628, using his fleet to make landfalls along the occupied coastlines, and destroying the naval facilities in Ålborg, Wismar, and Greifswald.