There isn't. At the back of Monaco is Beausoleil, which is France. I lived there for a year 1972/3 (I believe I virtually spent the year in a night club)
The castle is on an island. Even though the castle dominates the harbour area of the main town on this island, I had the whole place to myself...well, nearly.
@Legs Akimbo I will back track, it is considered part of Syracuse, its the island of Ortygia . Seperated by a bridge.