This has been done all the time in history!
Navy guns were placed on carriages durring the napoleonic war, the American Civil War, WWI <the rail guns> and WII, the large guns protecting Singapore, the guns from the damaged Battle Ships From Pearl Harbor were also used in fortifications in Hawaii, they also did this in the Philipines.
The Germans did this also on the Eastern Front, and in the Atlantic fortificaTIONS.
I have used them in some of my past campaigns. I liked using the old WWII 20mm antiaircraft guns as well as a 40mm Boffors on one platform.
A prime example is the creative use of the Germans 88mm flak as an antitank weapon.
I would think heavier guns could be mounted on trains, or entire gunturrets fitted on/in bunkers at key positions. Or maybe some freinkenarmor using lighter shipguns.
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