Actually, it is under construction now. Construction started in 1985 and the most possible date of construction's end is 200.
Bowser's Castle Project was invented by an unnamed Koopa Troopa, who was a farmer. Bowser and his army stopped near his farm one day and stole anything, that they could eat or sell. One of stolen things was a book with pictures of great castle, floating in space and firing lazors in Mario's house. Bowser said that he loves pictures and send the book to his castle. Later he send koopas to Space Sector 1337 to build the Castle.
Bowser's Castle is the biggest weapon warehouse in history. Most of the mass destruction weapons are in the castle dungeons.
UPDATE; Bush has been found in the basement of the castle, eating Toads that refused to marry him.