Règle correspondante
Practice Relating to Rule 37. Open Towns and Non-Defended Localities
Section A. Establishment of open towns
Switzerland’s Basic Military Manual (1987) notes that during the Second World War localities that were declared to be open were understood to be undefended should the enemy reach their periphery. It also points out different conditions that need to be fulfilled to obtain the status of “undefended areas”. 
Switzerland, Lois et coutumes de la guerre (Extrait et commentaire), Règlement 51.7/II f, Armée Suisse, 1987, Article 32.