Paris Observatory was funded in 1667 by king
Louis XIV.
The oldest building was designed by the architect
Claude Perrault.
It is the oldest observatory still serving. It is built without wood (to avoid fire) or metal (to avoid magnetic disturbances). It is a large rectangle (31 m x 29 m) with its four faces oriented with the
cardinal points of the compass. The
latitude of the south face defines the Paris latitude (48° 50' 11''). The
meridian line passing through its center defines the Paris
longitude.
The foundations are as deep (27 m) as high is the building itself. The basement is connected with the
Paris catacombs (visits forbidden). The catacombs consist of 65 km of underground galleries.
sources:
http://mesola.obspm.fr/gallery/paris/paris.html
http://www.obspm.fr/histoire/paris/paris.en.shtml