Some old paintings and drawings introduce secret routes to castles.

"A sketch by Leonardo da Vinci that shows Sforza Castle in the lower left (Image credit: Alamy)" (LiveScience, Mysterious tunnels sketched by Leonardo da Vinci in 1495 may finally have been discovered — hidden under a castle in Milan)

Researchers found the secret corridor in Milan. Leonardo da Vinci documented those corridors in his drawings. That means that there might be more secret chambers and passages hiding in old houses and castles. We know that Leonardo da Vinci hid some secret codes in his works.  

Sometimes. Painters hide secret passages in old paintings. The thing that looks like some kind of portrait can involve information about the secret corridors. 

That means. Things like Mona Lisa can involve some secret passages to some castles. Same way. Leonardo's Crossbow drawing can involve a secret passage to some castle. The problem is people don't know what the painting and castle are. 

Where the Giant Crossbow drawing belongs. The fact is that the painters must make those codes and encryption far away from the castle. If that kind of project is uncovered, that can cause problems for the painter and the map man. 

"Design for a Giant Crossbow - by Leonardo da Vinci" (Leonardo.net)

Smithsonian magazine

The thing. What makes it hard to solve those cryptical codes is that the map itself is drawn on paper that must be put on the right paintings. The thing outlines of the characters show the secret passages. But the problem is those maps. Those who made use of pairs with those paintings are lost. So, the painting and map are made as a pair. And the map was made precisely delivered with certain paintings. Without the map, people cannot find the passage. The painting is useless without maps. 

Researchers think that. Many famous paintings involve secret passages or routes to secret rooms like secret treasure chambers or secret meeting rooms. And they introduced passages in and out of castles. There are also many other codes hidden in famous paintings. Another thing is that the painter and map maker didn't always know the castle that those things were made for. The location of those castles and mansions was not the painter's and mapman's business. 

https://www.livescience.com/archaeology/mysterious-tunnels-sketched-by-leonardo-da-vinci-in-1495-may-finally-have-been-discovered-hidden-under-a-castle-in-milan

https://www.smithsonianmag.com/smart-news/secret-passageways-recorded-in-leonardo-da-vincis-sketches-discovered-beneath-a-medieval-castle-in-milan-180985890/

https://www.leonardodavinci.net/design-for-a-giant-crossbow.jsp