Frontier societies make for fascinating study. As an MA student I was mooting studying either Languedoc or the Christian/Muslim Iberian frontier in comparison to the Welsh Marches in the twelfth- and thirteenth-centuries before settling on studying pura Wallia (ie. beyond the frontier but on the Welsh side).

A primary source which would interest you is the Code of Cuenca. This was a municipal code for a frontier town on the Castilian border which codified the different judicial, administrative, and military functions of a town with three distinct religious groups (Christian, Muslim, and Jewish); of course as Cuenca lay on the Christian side of the 'frontier' the Christians generally came off slightly better. It is notable that the town was fiercely protective of its rights against the impositions of the local aristocracy (who would not have lived within the town for the most part) and that each faith-group was expected to defend the town against aggressors no matter what faith-group was threatening it.