While I had heard rumours of this in the past, Jeff Berry was kind enough to confirm the origins of Castle Blackmoor:
Seriously; let's never lose sight of the fact that the original 'rust monster' was a plastic toy that Dave got in a set of equally cheap plastic dinosaurs, and it's lasted for how long? You can still buy a copy of Dave's original Blackmoor Castle, too; it's an N scale model kit made by Kibri in Germany, their "Branzoll Castle" catalog number 7304. He used it for years to show people what the place looked like.
The real Branzoll Castle is located near the town of Chiusa (Klausen) in a formerly Austrian part of Northern Italy. The Castle was built around 1250. In 1671 it was left in ruins after a fire, but was rebuilt in 1895 by a castle-enthusiast by the name of Dr. Otto Piper. You can read more about the Castle here.
Below is a picture of the model from Kibri. Guess what I want for Christmas this year?