Since antiquity, sappers have used mining against a walled city, fortress, castle or other strongly held and fortified military position. Defenders have dug counter-mines to attack miners or destroy a mine threatening their fortifications. Since tunnels are commonplace in urban areas, tunnel warfare is often a feature, though usually a minor one, of urban warfare. A good example of this was seen in the Syrian Civil War in Aleppo, where in March 2015 rebels planted a large amount of explosives under the Syrian Air Force Intelligence Directorate headquarters.