Heaven is both a country (Luke 19:12; Heb. 11:14-16) and a city (Heb. 11:16; 12:22; 13:14; Rev. 21:12). A country is typically a large territory of various geographies, with citizens of diverse cultures and vocations, sometimes even languages, under one government that provides a common identity. A city is a place of many residences in near proximity. A city’s inhabitants are subject to the common government. Cities usually have varied and bustling activity, community events, education, arts, and visitors.
Heaven is and will be a place of great beauty, both natural created beauty and architecture, including streets of gold and buildings of pearls and emeralds and precious stones (Rev. 21:19-21). Heaven will have the advantages we associate with earthly cities, without the disadvantages (e.g. crime, pollution, corruption).
Heaven’s gates will always be open. People will travel in and out, some bringing treasures into the city (Rev. 21:24-25; 22:14). Travel outside the city shows that the city is not the whole of heaven, but merely its center. The great city is the capital of an endless empire, called a heavenly country (Heb. 11:16). There is a universe outside the city’s gates, to which its citizens have free access. Cities are characterized by visitors coming in and occupants going out for various reasons.
For more information on this subject, see Randy Alcorn's book Heaven.