Forts of wood were of occasional employment, the materials of which were transported from place to place, so that the structure might be speedily raised. Wace gives us a description of that brought over by William the Conqueror, and built up at Hastings:--
"Donc ont des nés mairrien[253] geté, A la terre l'ont traïné, Trestut percié è tut dolé: Li cheviles tutes dolées Orent _en granz bariz_ portées: Ainz ke il fust bien avespré, En ont un chastelet fermé."--_Line_ 11,658.
Mines were in use both by Richard I. and Philippe Auguste. At the siege of Acre in 1191, Richard attacked the city with archers and balistæ: "But more important than these," adds Devizes, "were the miners, making themselves a way beneath the ground, sapping the foundation of the walls, while soldiers bearing shields, having planted ladders, sought an entrance over the ramparts." The French king employed the mine at the siege of the Castle of Boves, as we learn from William the Breton. See also Rigord, page 185. The mines of these days were large caverns in which pillars of wood supported the incumbent mass. The posts being smeared with pitch and surrounded with combustibles, fire was then brought, and the stanchions being consumed, the walls fell in. With the mine came the counter-mine; an example of which occurs in the description by Guillaume le Breton of the siege of Château-Gaillard; where the English, countermining against the French, met them in their works and drove them back with slaughter:--
"Suffodiunt murum. Sed non minus hostis ab illâ Parte minare studet factoque foramine nostros Retrò minatores telis compellit abire."--_Philipp._, lib. vii.
Later, challenges were made, to be fought out in the mines, the combatants contending over a barrier of wood fixed in the midst. And Upton tells us that the aspirant to knighthood in a besieging army, no church being at hand, performed in the mine his vigil of arms.
While the besieging force plied their attack by means of the engines and mines already noticed, they had begun, in imitation of the ancients, to construct lines of circumvallation; in order at once to cut off the citizens from all communication with the open country, and to defend themselves against the sorties of the town. An example of this may be seen in the siege of Crema by the Emperor Frederic in 1159[254].