Type 'easy' or 'hard' to begin the quiz. Easy is a 15-degree grid, hard is a 10-degree grid. Then, try to name the SECOND LARGEST city in each grid square. To name a city, you must first click on the grid square to make it have a GREEN border. Then, type in the city answer for that square followed by a '.' (a period). You are given a score from 0 to 100 based on your answer's population compared to the 2nd largest city accepted in that square (100 being you named the 2nd largest city). If you guess the largest city it will be based on the ratio of the 2nd largest city's population to the largest.
At the end of the quiz the 2nd largest city will show in each unanswered square, and for those answered the 2nd largest city will be listed next to your score if you didn't answer it.
Quiz Takers 
Is there any reason I’m not aware of why Tromsø isn’t included? Current population looks to be over 50k but that might be a similar situation to Muscat I don’t know of.
You might be looking at the full municipality – city proper is a little too low:
https://citypopulation.de/en/norway/cities/ua/?cityid=8252
Saigon (Ho Chi Minh) should be accepted under both names as they’re equally used and thus should be on both the 6 and 9 letter quizzes.
Also, since Mumbai/Bombay is a 6 letter city no matter which of those two names you use, both should be accepted.
On most quizzes Saigon is accepted as an alternative for Ho Chi Minh City but for this letter-based series it must be one or the other as no city should appear on more than one quiz. As the official name for the city is Ho Chi Minh City that name should prevail over Saigon which, as useful as it is as an alternative name, it’s an outdated designation.