The Ms were the ones on wheels, they didn't need a concrete base. The 210s were capable of destroying all structures on the surface of a fort, but couldn't penetrate to the subterran casemates. The first fort to give up, de Barchon on August 8th, was only shelled by 210s and smaller guns.
The two Ms first opened fire in the evening of August 12th. - The Ms had not been completely tested when the war broke out. The battery (Kurze Marine Kanonen Batterie 3 - KMK 3) was formed at Krupp's at Essen starting on August 5th, 1914. On August 9/10th, they were put on a train and travelled to the Belgo-German border. From the border station they conducted a road march to Liège, then had to assemble their guns. In fact, the Ms only shelled two Liège forts, Pontisse and Loncin, all other forts were subdued by 210s and 150s.