Is there a MILS-like standard for medieval villages and castles?
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.