The Basques of each district kept their own defence provision, with men being drafted for the militias exclusively in defence of each specific district. However, voluntary military contribution to the king could go beyond district boundaries in exchange for a sum of money. The four districts kept a strong municipality based governmental structure, as well as minor customs on their boundaries and major ones on the Ebro river. Mineral extraction was concerted for communal exploitation, usually undertaken when required by neighbouring inhabitants and/or manufacturers (ironworks).