If it was necessary to seize control of a castle an army could either launch an assault or lay siege. It was more efficient to starve the garrison out than to assault it, particularly for the most heavily defended sites. Without relief from an external source, the defenders would eventually submit. Sieges could last weeks, months, and in rare cases years if the supplies of food and water were plentiful. A long siege could slow down the army, allowing help to come or for the enemy to prepare a larger force for later. Such an approach was not confined to castles, but was also applied to the fortified towns of the day. On occasion, siege castles would be built to defend the besiegers from a sudden sally and would have been abandoned after the siege ended one way or another.