The Line of Torres Vedras still preserves some of the wind mills and fortress that remain from a military defensive and observation system, constructed to the north of Lisbon between 1809 and 1810. Under a cloak of secrecy, the future Duke of Wellington developed a defence strategy based on the fortification of key points on the hilltops so as to be able to observe the access routes to the capital of Portugal and reinforce the natural obstacles of the local terrain. This system comprised three lines of defence, extending over 85 km from the Atlantic to the river Tagus.