The first key objective was the weak Spanish outworks on Monte Torrero. The hornwork and crownwork are subtypes of outworks. In this situation, the French increasingly relied on buried mines within threatened outworks. On 16 January 1809 the main Spanish outworks were in French hands. Between 1860 and 1874 outworks were added. Instead of immediately storming the two weak outworks, Murray insisted on establishing breaching batteries. The fort was surrounded by two wet ditches (moats) with extensive outworks between the ditches. The walls were surrounded by a number of outworks, consisting of hornworks, ravelins and counterguards. The bastions were separated by chevron-shaped outworks called "tenailles" located above the ditch. Parts of the walls, especially the outworks, were demolished to make way for urban development. Meanwhile, the besiegers overran the outworks one by one. In medieval Europe, numerous river crossings were protected by tower structures and outworks. Some outworks were subsequently added to reinforce the Royal Walls. The outworks are surrounded by a ditch, a covertway and a glacis. Whenever there was an interval in the fighting, the Spanish added more outworks.