City of London + City of Vatican + City of Columbia are the 3 independent states within states which composes the empire of the city

"A city-state is a region controlled exclusively by a city, usually having sovereignty." "Whereas the nation-states rely on invented or common heritage — commonly linguistic, historical, religious, economic, etc — the city-state relies on the common interest in the well functioning urban center. The urban center and its activity supplies the livelihoods of all urbanites inhabiting the city-state. Today, Singapore, Monaco and the Vatican City are the only sovereign states which bear any resemblance to the classical definition of a city-state. Several sovereign countries have self-governing areas delineated around cities. The typical case are nations with a federal administrative structure that have granted their capitals a special status outside that structure; examples are Washington, D.C. (coterminous with the District of Columbia) in the United States, Brasília (coterminous with the Brazilian Federal District), Mexico City (being the Mexican Federal District) and Canberra (part of the Australian Capital Territory). In nations without a federal administrative structure, capital cities sometimes enjoy a greater degree of autonomy, e.g. London. In Germany, Berlin has been a federal state (Land) of its own since 1990, but, unlike the above, remains inside the general administrative structure. (Note but this has nothing to do with its being the capital; the cities of Hamburg and Bremen have an identical status and look back on a much longer history of political independence.) A similar situation can be found in Austria, where the capital Vienna has the status of a state within the Austrian federation of states. Yet another case are Hong Kong and Macau, currently Special Administrative Region of the People's Republic of China. Neither capitals nor, arguably, even cities, their temporary, relative political independence results from agreements between China and the colonial powers that formerly controlled them." "4 Contemporary city-states 4.1 Sovereign city-states 4.1.1 Monaco 4.1.2 Singapore 4.1.3 Vatican City 4.2 Non-sovereign city states 4.2.1 City of London 4.2.2 Washington, D.C. 4.2.3 Other examples" "- Vatican City Until 1870, the city of Rome had been controlled by the pope as part of his "papal states". When King Victor Emmanuel II annexed the city in 1870, Pope Pius IX refused to recognize the newly-formed Kingdom of Italy. Because he could not travel through a place that he did not admit existed, Pius IX and his successors each claimed to be a "Prisoner in the Vatican", unable to leave the 0.44 km² (0.17-square mile) papal enclave once they had ascended the papal throne. The impasse was resolved in 1929 by the Lateran Treaties negotiated by the Italian dictator Benito Mussolini between King Victor Emmanuel III and Pope Pius XI. Under this treaty, the Vatican was recognized as an independent state, with the pope as its head. The Vatican City State has its own citizenship, diplomatic corps, flag, and postal system. With a population of less than 1000, it is by far the smallest sovereign country in the world, and widely recognized internationally as such. Non-sovereign city states - City of London See also: City of London Corporation Although the City of London is not commonly considered a city-state, it does have a unique political status (sui generis), a legacy of its uninterrupted integrity as a corporate city since the Anglo Saxon period and its singular relationship with the Crown. Historically its system of government was not unusual, but it was not reformed by the Municipal Reform Act 1835. It is administered by the City of London Corporation, headed by the Lord Mayor of the City of London (not the same post as the more recent Mayor of London, who presides over Greater London). The City is a ceremonial county too, although instead of having its own Lord-Lieutenant, the City of London has a Commission, headed by the Lord Mayor, exercising this function. - Washington, D.C. Not being part of any U.S. state, Washington, D.C.'s government operates as a city and state combined, although it is not a US state of its own." Source and further information: http://en.wikipedia.org/wiki/City-state