the choice of groups of buildings noteworthy for their purity of style , for the concentrations of monuments they contain and sometimes for their important historical associations . It is important for urban archaeological sites to be listed as integral units . A cluster of monuments or a small group of buildings is not adequate to suggest the multiple and complex functions of a city which have disappeared . Remains of such a city should be preserved in their entirety together with their natural surroundings whenever possible .
Inhabited historic towns In the case of inhabited historic towns , the difficulties are numerous , largely owing to the fragility of their urban fabric ( which has in many cases been seriously disrupted since the advent of the industrial era ) and the runaway speed with which their surroundings have been urbanised . To qualify for inscription , these towns should compel recognition because of their architectural interest and they should not be considered only on the intellectual grounds of the role that they may have played in the past or their value as historical symbols . To be eligible for Inscription on the World Heritage List , the spatial organisation , structure , materials , forms and , where possible , functions of a group of buildings should essentially reflect the civilisation or succession of civilisations which have prompted the nomination of the property . Four categories can be distinguished : a . Towns which are typical of a specific period or culture , which have been almost wholly preserved and which have remained largely unaffected by subsequent developments . Here the property to be listed is the entire town together with its surroundings , which must also be protected ; b . Towns that have evolved along characteristic lines and have been preserved , sometimes in the midst of exceptional natural surroundings , spatial arrangements and structures that are typical of the successive stages in their history . Here the clearly defined historic part takes precedence over the contemporary environment ; c . “ Historic centres ” that cover exactly the same area as ancient towns and are now enclosed within modern cities . Here it is necessary to determine the precise limits of the property in its widest historical dimensions and to make appropriate provision for its immediate surroundings ; and d . Sectors , areas or isolated units which , even in the residual state in which they have survived , provide coherent evidence of the character of a historic town which has disappeared . In such cases surviving areas and buildings should bear sufficient testimony to the former whole .
Historic centres and historic areas should be listed only when they contain a large number of ancient buildings of monumental importance which provide a direct indication of the characteristic features of a town of exceptional interest . Nominations of several isolated and unrelated buildings which allegedly represent , in themselves , a town whose urban fabric has ceased to be discernible , should not be encouraged .
However , nominations could be made regarding properties that occupy a limited space but have had a major influence on the history of town planning . In such cases , the nomination should make it clear that it is the monumental group that is to be listed and that the town is mentioned only incidentally as the place where the property is located . Similarly , if a building of clearly Outstanding Universal Value is located in severely degraded or insufficiently representative urban surroundings , it should , of course , be listed without any special reference to the town .
New towns of the 20 th century It is difficult to assess the quality of new towns of the 20 th century . History alone will tell which of them will best serve as examples of contemporary town planning . The examination of the files on these towns should be deferred , save under exceptional circumstances . Under present conditions , preference should be given to the inscription on the World Heritage List of small or medium-sized urban areas which are in a position to manage any potential growth , rather than the great metropolises , on which sufficiently complete information and documentation cannot readily be provided that would serve as a satisfactory basis for their inscription in their entirety .
In view of the effects which the inscription of a town on the World Heritage List could have on
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