Siege
Fortresses built in this style throughout the 16th century did not become fully obsolete until the 19th century, and were still in use throughout World War I (though modified for 20th-century warfare). During World War II, trace italienne fortresses could still present a formidable challenge, for example, in the last days of World War II, during the Battle in Berlin, that saw some of the heaviest urban fighting of the war, the Soviets did not attempt to storm the Spandau Citadel (built between 1559 and 1594), but chose to invest it and negotiate its surrender.
However, the cost of building such vast modern fortifications was incredibly high, and was often too much for individual cities to undertake. Many were bankrupted in the process of building them; others, such as Siena, spent so much money on fortifications that they were unable to maintain their armies properly, and so lost their wars anyway. Nonetheless, innumerable large and impressive fortresses were built throughout northern Italy in the first decades of the 16th century to resist repeated French invasions that became known as the Italian Wars. Many stand to this day.
In the 1530s and 1540s, the new style of fortification began to spread out of Italy into the rest of Europe, particularly to France, the Netherlands, and Spain. Italian engineers were in enormous demand throughout Europe, especially in war-torn areas such as the Netherlands, which became dotted by towns encircled in modern fortifications. The densely populated areas of Northern Italy and the United Provinces (the Netherlands) were infamous for their high degree of fortification of cities. It made campaigns in these areas very hard to successfully conduct, considering even minor cities had to be captured by siege within the span of the campaigning season. In the Dutch case, the possibility of flooding large parts of the land provided an additional obstacle to besiegers, for example at the Siege of Leiden. For many years, defensive and offensive tactics were well balanced, leading to protracted and costly wars such as Europe had never known, involving more and more planning and government involvement. The new fortresses ensured that war rarely extended beyond a series of sieges. Because the new fortresses could easily hold 10,000 men, an attacking army could not ignore a powerfully fortified position without serious risk of counterattack. As a result, virtually all towns had to be taken, and that was usually a long, drawn-out affair, potentially lasting from several months to years, while the members of the town were starved to death. Most battles in this period were between besieging armies and relief columns sent to rescue the besieged.

Marshal Vauban and Van Coehoorn: At the end of 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 defenses 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 defenses: 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 metres (2,000 ft) 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 metres (1,000 ft). This line contained smaller guns. The final line would be constructed only 30 to 60 metres (100 to 200 ft) 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 in sieges until the 19th century. It would also be from this line that miners working to undermine the fortress would operate.
The trenches connecting the various lines of the besiegers could not be built perpendicular to the walls of the fortress, as the defenders would have a clear line of fire along the whole trench. Thus, these lines (known as saps) needed to be sharply jagged.

Another element of a fortress was the citadel. Usually, a citadel was a "mini fortress" within the larger fortress, sometimes designed as a reduit, but more often as a means of protecting the garrison from potential revolt in the city. The citadel was used in wartime and peacetime to keep the residents of the city in line.
As in ages past, most sieges were decided with very little fighting between the opposing armies. An attacker's army was poorly served, incurring the high casualties that a direct assault on a fortress would entail. Usually, they would wait until supplies inside the fortifications were exhausted or disease had weakened the defenders to the point that they were willing to surrender. At the same time, diseases, especially typhus, were a constant danger to the encamped armies outside the fortress, and often forced a premature retreat. Sieges were often won by the army that lasted the longest.
An important element of strategy for the besieging army was whether or not to allow the encamped city to surrender. Usually, it was preferable to graciously allow a surrender, both to save on casualties, and to set an example for future defending cities. A city that was allowed to surrender with minimal loss of life was much better off than a city that held out for a long time and was brutally butchered at the end. Moreover, if an attacking army had a reputation of killing and pillaging regardless of a surrender, then other cities' defensive efforts would be redoubled. Usually, a city would surrender (with no honour lost) when its inner lines of defense were reached by the attacker. In case of refusal, however, the inner lines would have to be stormed by the attacker and the attacking troops would be seen to be justified in sacking the city.

Siege warfare: Siege warfare dominated in Western Europe for most of the 17th and 18th centuries. An entire campaign, or longer, could be used in a single siege (for example, Ostend in 1601–1604; La Rochelle in 1627–1628). This resulted in extremely prolonged conflicts. The balance was that, while siege warfare was extremely expensive and very slow, it was very successful—or, at least, more so than encounters in the field. Battles arose through clashes between besiegers and relieving armies, but the principle was a slow, grinding victory by the greater economic power. The relatively rare attempts at forcing pitched battles (Gustavus Adolphus in 1630; the French against the Dutch in 1672 or 1688) were almost always expensive failures.

The exception to this rule were the English. During the English Civil War, anything which tended to prolong the struggle, or seemed like want of energy and avoidance of a decision, was bitterly resented by the men of both sides. In France and Germany, the prolongation of a war meant continued employment for the soldiers, but in England, both sides were looking to end the war quickly. Even when in the end the New Model Army—a regular professional army—developed the original decision-compelling spirit permeated the whole organisation, as was seen when pitched against regular professional continental troops the Battle of the Dunes during the Interregnum.

Experienced commanders on both sides in the English Civil War recommended the abandonment of garrisoned fortifications for two primary reasons. The first, as for example proposed by the Royalist Sir Richard Willis to King Charles, was that by abandoning the garrisoning of all but the most strategic locations in one's own territory, far more troops would be available for the field armies, and it was the field armies which would decide the conflict. The other argument was that by slighting potential strong points in one's own territory, an enemy expeditionary force, or local enemy rising, would find it more difficult to consolidate territorial gains against an inevitable counterattack.