Sieges became a prominent part of the war with over 300 occurring during the period, many of them involving castles. Indeed, as Robert Liddiard suggests, the "military role of some castles in the seventeenth century is out of all proportion to their medieval histories". Artillery formed an essential part of these sieges, with the "characteristic military action" according to military historian Stephen Bull, being "an attack on a fortified strongpoint" supported by artillery. The ratio of artillery pieces to defenders varied considerably in sieges, but in all cases there were more guns than in previous conflicts; up to one artillery piece for every nine defenders was not unknown in extreme cases, such as near Pendennis Castle. The growth in the number and size of siege artillery favoured those who had the resources to purchase and deploy these weapons. Artillery had improved by the 1640s but was still not always decisive, as the lighter cannon of the period found it hard to penetrate earth and timber bulwarks and defences – demonstrated in the siege of Corfe. Mortars, able to lob fire over the taller walls, proved particularly effective against castles – in particular those more compact ones with smaller courtyards and open areas, such as at Stirling Castle.