Leros is a specific interest (which is why y'all will eventually see an R-Boat build), and somewhere I have an annotated map of the Italian gun positions on Leros' heights. They were mostly WWI rejects, nothing as massive (or as cinematically engaging) as the twin railroad cannons in the movie. In reality, the amount of effort to set two of those main guns in a mountaintop emplacement would be insane (not that the Nazis weren't nuts to start with).
In reality, the Italian guns never controlled any important shipping lanes which is why this project is more malleable - the items are the sort of wargaming terrain that would allow small-scale naval gamers to side-step into 20mm commando raids; Russian & Japanese (1905 Siberia), Japanese vs pre-Dreadnaught America, Commonwealth or Russians vs Germans (Baltic 1914-1917), Japanese & Commonwealth (Singapore 1941), Commonwealth vs Italians (1940 Asmara), Italians vs Germans (Leros-like 1943), ad nauseum (the more obscure, the better).
As a wargaming tool, large guns would only be a target in a 20mm game but if combined with a naval game, they'd probably be engaging targets on a different map.
Funfact: as I understand it, the tubes used to model the guns in the movie are on display at a beach in Senegal.
nb: This might help visualize *why* this thread will contain different sort of cannons.
Note that the donor kits - made by several manufacturers over the last decade or two - are out of production now
Beat up/broken/missing parts examples can be found in the usual places.