the 17th century, two influential military engineers, the French Marshal Vauban and the Dutch military engineer Menno van Coehoorn, developed modern fortification to its pinnacle, refining siege warfare without fundamentally altering it: ditches would be dug; walls would be protected by glacis; and bastions would enfilade an attacker. Both engineers developed their ideas independently, but came to similar general rules regarding defensive construction and offensive action against fortifications. Both were skilled in conducting sieges and defences themselves. Before Vauban and Van Coehoorn, sieges had been somewhat slapdash operations. Vauban and Van Coehoorn refined besieging to a science with a methodical process that, if uninterrupted, would break even the strongest fortifications. Examples of their styles of fortifications are Arras (Vauban) and the no-longer-existent fortress of Bergen op Zoom (Van Coehoorn). The main differences between the two lay in the difference in terrain on which Vauban and Van Coehoorn constructed their defences: Vauban in the sometimes more hilly and mountainous terrain of France, Van Coehoorn in the flat and floodable lowlands of the Netherlands. Planning and maintaining a siege is just as difficult as fending one off. A besieging army must be prepared to repel both sorties from the besieged area and also any attack that may try to relieve the defenders. It was thus usual to construct lines of trenches and defenses facing in both directions. The outermost lines, known as the lines of contravallation, would surround the entire besieging army and protect it from attackers. This would be the first construction effort of a besieging army, built soon after a fortress or city had been invested. A line of circumvallation would also be constructed, facing in towards the besieged area, to protect against sorties by the defenders and to prevent the besieged from escaping. The next line, which Vauban usually placed at about 600 meters from the target, would contain the main batteries of heavy cannons so that they could hit the target without being vulnerable themselves. Once this line was established, work crews would move forward, creating another line at 250 meters. This line contained smaller guns. The final line would be constructed only 30 to 60 meters from the fortress. This line would contain the mortars and would act as a staging area for attack parties once the walls were breached. Van Coehoorn developed a small and easily movable mortar named the coehorn, variations of which were used