One of the reasons for warfare's increased impact was its indecisiveness. Armies were slow moving in an era before good roads and canals. Battles were relatively rare as armies could manoeuvre for months, with no direct conflict. In addition, battles were often made irrelevant by the proliferation of advanced, bastioned fortifications. To control an area, armies had to take fortified towns, regardless of whether they defeated their enemies' field armies. As a result, by far the most common battles of the era were sieges, hugely time-consuming and expensive affairs. Storming a fortified city could result in massive casualties and cities which did not surrender before an assault were usually brutally sacked -for example Magdeburg in 1631 or Drogheda in 1649. In addition, both garrisons and besiegers often suffered heavily from disease.