Related Rule
United Kingdom of Great Britain and Northern Ireland
Practice Relating to Rule 37. Open Towns and Non-Defended Localities
Section A. Establishment of open towns
The UK Military Manual (1958) defines an open or undefended town as:
A town which is so completely undefended from within or without that the enemy may enter and take possession of it without fighting or incurring casualties. It follows that no town located behind the immediate front line can be deemed open or undefended, since the attacker must fight his way to it. Any town behind the enemy front line is thus a defended town and is open to ground or other bombardment, subject to the conditions imposed on all bombardment, namely, that as far as possible, the latter must be limited to military objectives … A town in the front line with no means of defence, not defended from the outside and into which the enemy may enter and of which he may take possession at any time without fighting or incurring casualties, e.g., from crossing unmarked minefields, is undefended even if it contains munitions factories. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 290.
The manual goes on to say that, prima facie, a fortified place is considered as defended, unless there are visible signs of surrender. However, a locality need not be fortified to be deemed “defended”, and it may be held thus if a military force is occupying it or marching through it. It states that a town should be considered to be defended (and thus liable to bombardment) even if defended posts are detached and located at a distance from the city:
The town and defended posts form an indivisible whole, inasmuch as the town may contain workshops and provide supplies which are invaluable to the defence and may serve to shelter the troops holding the defence points when they are not on duty. 
United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 289.