I'm aware of the MILS standard for building large layouts including city and train elements:
This could obviously be adapted for use in medieval layouts, but is there something like this that has already been standardized and put in use by LUGs?
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The Modular Medieval Village Standard put together by the Rochester, NY LUG (RochLUG) provides this. It features an underlying support layer very similar to MILS:
Most modules will feature a randomized cobblestone road providing transit between structures:
Structures of various kinds can be added to the modules:
Many modules created by different people can be connected to create convincing medieval villages:
Complete details of the system can be found in the PDF shared by RochLUG.