Related Rule
Belgium
Practice Relating to Rule 37. Open Towns and Non-Defended Localities
Belgium’s Law of War Manual (1983) states: “an area is considered as an ‘undefended area’ or as an ‘open town’ when it is undefended to the point that it can be taken without a single shot or without any losses (e.g. due to the presence of mines)”. It adds that the presence of wounded military personnel and weapons does not change the status of the area as an open town or undefended area. The manual points out two procedures to obtain the status of “open town”, namely, a unilateral declaration or an agreement between the belligerents. 
Belgium, Droit Pénal et Disciplinaire Militaire et Droit de la Guerre, Deuxième Partie, Droit de la Guerre, Ecole Royale Militaire, par J. Maes, Chargé de cours, Avocat-général près la Cour Militaire, D/1983/1187/029, 1983, p. 30.
Belgium’s Teaching Manual for Soldiers contains a slide illustrating the prohibition of bombardment of a village in which no combatants or military objects are located. 
Belgium, Droit de la Guerre, Dossier d’Instruction pour Soldat, à l’attention des officiers instructeurs, JS3, Etat-Major Général, Forces Armées belges, undated, p. 86 and slide 7/2.
Belgium’s Law concerning the Repression of Grave Breaches of the Geneva Conventions and their Additional Protocols (1993), as amended in 1999, provides that “making non-defended localities … the object of attack” constitutes a crime under international law. 
Belgium, Law concerning the Repression of Grave Breaches of the Geneva Conventions and their Additional Protocols, 1993, as amended in 1999, Article 1(14).
Belgium’s Penal Code (1867), as amended in 2003, provides:
War crimes envisaged in the 1949 [Geneva] Conventions … and in the [1977 Additional Protocols I and II] … , as well as in Article 8(2)(f) of the [1998 ICC Statute], and listed below, … constitute crimes under international law and shall be punished in accordance with the provisions of the present title … :
24. making … towns, villages, dwellings, or buildings which are undefended and which are not military objectives … the object of attack or bombardment, by whatever means. 
Belgium, Penal Code, 1867, as amended on 5 August 2003, Chapter III, Title I bis, Article 136 quater, § 1(24).
Belgium’s Law relating to the Repression of Grave Breaches of International Humanitarian Law (1993), as amended in 2003, provides:
War crimes envisaged in the 1949 [Geneva] Conventions … and in the [1977 Additional Protocols I and II] …, as well as in Article 8(2)(f) of the [1998 ICC Statute], and listed below, … constitute crimes under international law and shall be punished in accordance with the provisions of the present title … :
14. making … towns, villages, dwellings, or buildings which are undefended and which are not military objectives … the object of attack or bombardment, by whatever means. 
Belgium, Law relating to the Repression of Grave Breaches of International Humanitarian Law, 1993, as amended on 23 April 2003, Article 1 ter, § 1(14).