Originally the home of Cardinal Richelieu, it was built in the 1630s and after the Cardinal's death fell into the hands of King Louis XIII. Today it is the location of the Ministry of Culture and a branch of the National Library.
This was a great way to see the sites of Paris. I'd definitely recommend the 2 day option. Another company only offers 1 route, rather than 4, so choose wisely. The only downside was that it was sometimes hard to find the bus stops - even with a map.
Hands down the best investment! Do this to get around Paris!