Winston Stark designed a veritable castle fortress with arms of all sorts, including nuclear-powered rockets, barely ten minutes away from FatLand by air. Air was the easiest way to get there, but people were invited by Stark. It was not open to the public.
FIN (FatLand Intelligence Network) knew of Stark's fortress soon after it was built, but they also knew that he would never actually fire on FatLand. He used the idea of the weapons - his own WMD, so to speak- to deter action by FatLand against him for his questionable business deals and practices. Occasionally he did send drones over FatLand to intimidate the FatLand Board, but did not do more.
Margaret Clancy, whom Stark continued to woo, even after his second marriage (to a Scandinavian/Russian model and gymnast), was a prisoner in Stark's fortress for a short period of time and managed to gather some information about the weapons systems, which were then redone by Stark so that FatLand would not have access to them.