Saint Germain en Laye enters history with king Robert II the Pious, who commands the erection of a monastery at the beginning of the XIth century. In 1124 a royal residence is built, that it is enlarged to a large dimension in 1556, but today just some remains are still visible. Even if there is an interesting renaissance palace (Château neuf), the element that I think makes it worth a trip from Paris (half an hour by RER A) is the Grande Terrasse, an avenue 1950 meter long and 30 wide.
The Grande Terrasse faces the Saint Germain woods to the vest and allows for an unobstructed view to the east, with the Defense towers and, in some parts, the Tour Eiffel as main reference elements in the skyline.
Created by André Le Nôtre between 1669 and 1674, it is a space in which the usual strong geometry of the French gardens is limited to the platform itself (to the west the woods are not transformed, and to the east there is a steep slope), and the landscape takes the main lead, with one of the most open views in the Parisian region.