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The conditions of the Sierra, more fresh and humid than the plateaus, and its lesser transformation by human activity, have turned these mountains into a privileged refuge of biodiversity. In its physical environment, its cirques and glacial lagoons and its granite rocks stand out; between its vegetal landscapes, the high mountain ecosystems and the extensive pine forests of Siberian pine.
In the first centuries of the Reconquest, the entire Central System constituted a border territory, a frequent scene of skirmishes between Muslims and Christians. Surely at that time the area of the Sierra de Guadarrama should have been sparsely populated, as it was frequently ravaged by the combatants.
With the conquest of Toledo by Alfonso VI, in 1085, the Tagus River began to mark the new frontier between the two territories, although for some decades the Muslim raids in the Sierra were still important.
The city of Segovia soon stood out as a key center in the process of reconquest and repopulation. As the Muslim incursions became less frequent, the military role of the city began to have its replication in the economic-productive field. The livestock activity had a special relevance from the start. The possibilities of mobility that the herds had turned to the cattle ranch in the productive activity better adapted to the instability caused by the border conflicts. In addition, the pastures of the mountain peaks, available in the summer, constituted an excellent complement to the pastures of the lands of the plateau, which were exhausted precisely in the summer.
Although a large part of the history of the Guadarrama is only a few archaeological remains, in the XIV century there are two written works that show us the mountain geography fairly accurately. One is the "Book of Good Love" of the Archpriest of Hita, with its references to the mountain passages, trades and customs, and the other the "Book of Montería", by Alfonso XI, which he describes with great precision and meticulousness , abundant terms and places of these mountains, related to the big game. Precisely it will be the hunting activity, together with the amenity of the landscapes, the reason that encourages the monarchs to build country houses and pleasure palaces in the Sierra. Austrias and Bourbons with the palatial buildings of Valsaín, San Lorenzo, San Ildefonso or Riofrío contributed an exclusive richness to the artistic patrimony of this area.
S XVIII brought significant improvements in communications between the two mountain slopes. The new Bourbon dynasty, in full Enlightenment, will promote projects such as Fernando VI, which commissioned the current layout of the port of Guadarrama (1,511 m), or that of his successor, Carlos III, who did with the port of Navacerrada ( 1858 m) to replace the historic Fuenfría Pass.
Given its barrier nature interposed between both plateaus, the Sierra de Guadarrama would be the scene of battles both in the War of Independence (Battle of Somosierra, 1808) and in the Spanish Civil War, a time when the Sierra marked the front that separated the Republican and national sides. Through all these mountains are still abundant remains of the last contest.
Since the end of the 19th century, another kind of approach to the mountains has been produced, which, favored by the new pedagogical currents promoted by the Free Institution of Education, will originate the discovery of these mountains from the perspective of the scientific movement. Naturalists, geologists, geographers, anthropologists ... will turn the Guadarrama into one of the best studied mountains in Spain.