Solid walls used as a defensive system were only vulnerable to assaults and undermining. They originally included small openings through which weapons could be fired, although these subsequently had to be enlarged to accommodate cannons. Tall towers flanked walled sections to defend against assaults, and a range of devices were used to hinder attackers: isolated albarrana towers standing outside (but connected to) the walls, a barbican wall running parallel to the main wall, but thinner and lower than it, a ditch between the two, and finally a gateway protected by towers which, from the 5th/11th century, included a dogleg entrance that forced attackers to make a right-angled turn.