Paris has been around for millennia; but it wasn't until 1605, when King Henry IV built what was then-called Place Royale, that a public square was planned into the city's landscape. It's now known as the Place des Vosges, and to this day remains largely unchanged since its inauguration in 1612.
It's easy to call any public area in a major city an “oasis,” but Place des Vosges truly lives up to the description. It's in Le Marais, which is already a relatively quiet arrondissement; but once you step through the arches, the stately residences seem to absorb any city noise and the arcades that cover the sidewalks add to its hushed ambiance. It's a good place to go to take a load off after trekking around the city all day.
The border of the third and fourth arrondissements divide the Place des Vosges; take the M7 to Sully-Morand, then head down Rue Beautrellis, which ends at the exterior of the homes that surround Place des Vosges.