Luxembourg Palace is the home of French Senate, and it was built between 1615 and 1645. It's possible to visit, free of charge, it in the third weekend of the month of September, when it happens the Open Gates of European Institutions.
The palace worth a waiting at a queue, having brilliant halls with painted ceilings, large libraries, rare paintings and statues.
Also you have the privilege to see the Senate Hall, where French senators debate important state problems.