It would be helpful to state the criteria you are using to define the size of a city as they vary widely. For example, you give Manila as 1.8 million whereas its Metro Area has a population of 13.5 million and on other measures it is as much as 24.9 million. This makes it very difficult to do this quiz.
Also, for most cities that end in 'city' we usually do not have to type the word city (Djibouti, Luxembourg, etc.)
Also, Gaborone, Skopje, Vatican, and Yaren are missing. And it would be great if you accepted cities like "Djibouti" without typing the "city".