Fortress of Mainz The Fortress of Mainz was a fortress ed garrison town between 1620 and 1918 . At the end of the Napoleonic Wars , under the term of the 1815 Peace of Paris , the control of Mainz passed to the German Confederation and became part of a chain of strategic fortresses which protected the Confederation . With the dissolution of the Confederation and the Austro - Prussian War , control of the fortress first passed to Prussia , and , after the 1871 Unification of Germany , to the German Empire . 1839 . In 1839 an article on Mainz in " The Penny Cyclopædia " stated that that Mainz was one of the strongest fortresses in Europe , and a chief bulwark of Germany against France . At the Congress of Vienna , Mainz was assigned to the Louis , Grand - Duke of Hesse - Darmstadt , but it was decided that , as a fortress , it should belong to the German Confederation , with a garrison of Austrian , Prussian , and Hessian troops . This garrison in time of peace consisted of 6 , 000 men . The military governor , who retained his post five years , was alternately an Austrian and a Prussian general . A criticism of the fortress was that it was too large , as it required for its defence a garrison of 30 , 000 men . The fortress of Mainz was connected , by a bridge over the Rhine , with the strongly fortified village of Kastel . The extent of the works , which were much enlarged by the French while the city was in their possession , including the work called the " Weisenauer Schanze " or " Fort Weisenau " , but exclusive of Kastel and of the small redoubt , was two and a half leagues . Among the principal works were the citadel , with the " Eichelstein " , and that called the " Hauptstein " , an extremely strong work projecting beyond all the rest , on an eminence called the " Linsenberg " . Kastel , which at that time was united with Mainz as an outwork , had very extensive fortifications , which consisted of four strong forts besides the strongly fortified island of Petersau , including which latter the works were of greater extent than even those of Mainz itself . The inner works consisted of 14 principal and 13 smaller bastions . On the land side there were four great gates with double drawbridges , and toward the river several more gates . The Rhine runs from south to north , and the Maine from east to west . About a mile above the junction of the two rivers was the village of Kostheim on the Maine , and a little farther up a bridge of boats , defended by a strong " tête - de - pont " . 1864 . Karl Baedeker writing in 1864 stated that Mainz was amongst the strongest fortresses of the German Confederation . It was surrounded by a threefold line of fortifications : first ring , the chief rampart consisting of 14 bastions comprising the citadel ; second ring , a line of advanced forts , connected by glacis ; third ring , by still more advanced entrenchments , erected partly by the Prussian , partly by the Austrian engineers , of which the principal were the " Weisenauer Lager " , the " Hartenberg " , and the " Binger Thurm " . On the north side of the town stood a vast Military Hospital , facing the " Schlossplatz " . In time of peace the garrison consisted of 3 , 000 Prussian , and a similar number of Austrian troops ; in time of war the number of soldiers could be trebled . Structures and buildings . According to Lehnhardts map of Mainz ~ 1844 many bastion s are to be found :