If you’re like me, you’re familiar with the Paris catacombs. They’re not the only such network of tunnels and many exist all over the world. Unlike limestone caverns, which are formed through the natural actions of nature, a large number of these tunnels were expressly for the purpose of mining limestone for the use of construction. As a result, they occur alongside populated areas like Paris, Maastricht, and Odessa.
An article in the Atlas Obscura discussed the entry for the Odessa Catacombs. Like the Paris catacombs, they were used to extract limestone, though apparently of a somewhat more recent vintage, dating back to the 1600s and with the greatest activity in the 1800s.
That anachronism aside, any section of the catacombs might be used by a gamemaster looking for a subterranean map. Even that might not be a stumbling block, given how many high-fantasy settings are littered with buildings owing more to the likes of Neuschwanstein Castle than to early medieval castles.