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 defence 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,