A 'Portal' is popularly defined as the web access to the support services of a buisness - industry - vertical integration.
A 'City' portal can be one of two things, either internal for the various departments and their personnel and would then contain Human Resources (benefits, jobs, daily news), and financials (budgeting, planning), and departmental sites to promote interaction and communications.
Or, an external operation between the government and its citizens. Once again you would integrate the various departments to expose the services offered and promote communications.
Because the requirements are so different (security, access limits, projected audience, etc.) I do not know of a sucessful portal that combines the two.
Define your audience, determine what questions that audience would have daily, weekly, monthly, annually; and provide the answers as easily as possible. That is the recipe for success. It is the details of providing answers to questions you don't know yet, that make it work.