During the Bishops' Wars of 1638-1640, the Maxwells supported Charles I. A garrison of up to 100 men was installed at Threave, and further earthworks were added to the Castle's defences. The army of the Covenanters, opposed to the royalist cause, arrived in summer 1640 and laid siege to the castle. After holding out for 13 weeks, the garrison surrendered on the orders of King Charles. The Covenanters ordered the buildings to be dismantled, and the materials to be disposed "to the use of the public".