This is mostly a lot of big leaps of logic, but I think Chateau de Meudon could be it?
First, it was decorated by Claude Audran III sometime between 1695 and 1711, which includes 1708, the year that Antoine Watteau began working with Audran, which is a potential connection to Watteau, especially if he happened to assist in decorating this place.
Second, it was bought in 1426 by the valet of Charles VII. Charles VII was the grandfather of Charles VIII, who Anne of Brittany was married to for 7 years. That’s a tenuous connection but the best I could find, really. The valet’s grandson, Antoine Sanguin, then demolished and rebuilt the manor in an Italian renaissance style in 1520. The place has been rebuilt several times which fits the “new buildings” part of Ali’s sense. Edit: Antoine Sanguin’s father, Jean Sanguin, was apparently known as the “Bastard of Sanguin,” although whether he was actually illegitimate is unknown and I couldn’t find anything about the man on the internet unfortunately.
Third, it sits between Paris and Versailles “in the heart of a hunting reserve”, according to wikipedia. Louis XVI loved to hunt here apparently.
Fourth, in 1793, Meudon was converted to a testing facility for military “aerostats” and other scientific experiments. In 1795, a fire destroyed much of certain parts of the chateau, caused by some of the research being conducted.
To be honest I’m not entirely sure where the illegitimate son or confluence part comes in? And some of this is pretty tenuous, but I suppose we could consider some of Louis XVI and Marie Antoinette’s children to be “illegitimate” in the antiquated sense because they were adopted? As far as I am aware though, neither of their adopted sons ever married or had children. Louis XV had a LOT of illegitimate kids, only one of whom was ever legitimized, so it also theoretically could have been one of them (although the legitimized son ended up becoming an Abbot so maybe didn’t have kids). A TON of people have lived here over the ages though so there’s a fairly decent chance that someone was illegitimate and having a good time! The confluence part is harder to understand though.