In the 16th century, the strategic importance of Maastricht becomes particulary apparent. In 1567 the city, so far having used its own resources in defending itself, relying on the citizen soldiery and organised parish militias, is forced to accommodate a permanent garrison under the command of a military governor appointed by Brussels.
Meanwhile, artillery arms had passed through a spectacular development. The medieval town wall provided insufficient protection from the new weapons, nor did it allow of effective use of the town's bombards, with their picturesque names like "Kwade Griet"( angrie Marge), 'Rode Hond" (red dog) en de "Maagd van Tricht "( Maastricht maid). The modernization of the fortress was effected initially at the instance of Brussels ( 1542-1555), next under the threat of a siege by the duke of Parma, which eventually took place in 1579. It comprised the reinforcement of the ring wall, by walling up the arches of the wall walk and raising a heavy bank against the inner side, the filling-up of the towers after they had been lowered tot the level of the wall, the demolation of the upper storeys of the gatehouses, and the replacement of the bettlements by a continuous parapet contructed in brck. This raised the defensive power of the enceinte considerably and provided the necessary space for mounting ordnance, on the walls and the truncated towers. The construction of low platforms for ordnance at the foot of some of the towers, and of polygonal ravelins in front of the gates made it now possible for the moat and the field side of the wall to be adequately covered. The ravelins formed small islands in the moat and hid the gates from the view and the fire of the enemy. The fall of the ravelin before the Brusselsepoort in 1579 decided the outcome of the siege. Shortly before 1579 also the sunterrenean mine gallery came to form part of the fortifications.The southern section was protected by a moat, and the whole area could, additionally, be innundated with water from the Jeker. There was another moat in front of the north wall, between the Lindenkruispoort and te river Maas. As in summer this moat was ussually dry, the Spanish garrison shortly after 1679 dug a deep ditch in its bottom between the Tongersepoort and the Lindenkruispoort, through which water from the Jeker could be conducted to the northern sector.
After the fortifications of the Nieuwstad the enceinte was, in all, about 4700 metres long. It ws thoroughly altered in the 16th century, to adapt is to the changed demands of warfare; later alterations were restricted mainly to repair of war damage and fortification of specially threatened points, such as the Brusselsepoort. What has remained of the second enceinte is particular the wall between the Sint-Pieterspoort and the Tongersepoort ( both demoloshed c. 1868) now forms the picturesque backdrop to a fine park for promedaders along the Jeker, but it conveys only an incomplete impression of the former city wall as it once stood, most of it high on a eartwork, with battlements, spires and gatehouses.