First choose any Europe 10K+ pop city by entering it and choosing it in the drop-down box. Then, name any Europe 10K+ city in each 100 km circle/ring as the rings expand outward from your city of choice, trying to name SECOND LARGEST city in each circle. The first circle will include your city of choice as a possible answer.
PLEASE NOTE - You must type a '.' (period) after each city to submit it to prevent answering another city with a name you would be have to type to get your answer. If multiple cities have the name you submitted, only the largest one will be counted.
You accumulate points based on the population of the city you named in relation to the second largest city in the circle. A smaller city’s point total will be the ratio of its population to the largest, so if you name a city of 75K people and the largest city has 100K, you get 75 points. Naming the largest city will get you points based on the ratio of the 2nd largest city's population to the largest city's.
When you run out of time or end the quiz, you will see the current ring’s 2nd largest city at right (under ‘Answer Stats’ by your coordinates).
For example, if you choose ‘Paris’ – you will be shown a circle of radius 100 km centered at Paris. Naming any city in this circle (including Paris) will cause another ‘ring’ of radius 100 km to appear outside that circle, however - you will want to try naming the SECOND LARGEST city you can to get more points. After completing that circle, you now need to name any 10K+ Europe city in the new ring (but not within any previous circles/rings), again trying to name the 2nd largest city. If there are not two cities within the next ring of that distance, the ring will keep expanding by that distance until two cities fall within it.
Quiz Takers 
Typing Moscow — and Mos in Portugal turns up first. Ensure you have to add full stops please to prevent this!
The biggest city in one square was Nicosia, and it also filled in some commune in Sicily with the same name! Have it so that you add full stops. Though that wouldn’t have resolved this particular issue!!
Problem is, you start typing a town like Brasov, and then Bra some town in Italy gets chosen, Maybe introduce having to add a full stop (period) to avoid this.
for some reason if you type Mora and then Quintanar it lets you have 2 cities in the same square
There are some problems in the the expert level quiz. Such as:-
All populations are from my source, citypopulation.de, and using city proper populations. I don’t know where you are getting some of these claims? Antwerp fills in a square, as does Ghent and Masstricht. The short city names are possibly part of a longer name where I accept a shortened version.
It is any city instead of biggest city. The color of the dots show that Maastricht is bigger than Stein for example. Also the towns of Son, Mol and Lent are all real. Son is one of the towns in Gemeente Son en Breugel since they are considered 2 different towns and Lent is next to Nijmegen.
Uusikapunki isn’t accepted for some reason
It’s Uusikaupunki
fun fact – kaupunki has the same origin as köping. Sweden-finland connection, both use köping for some cities.
needs more time on advanced level (and probably on expert too)
yeah tons of squares – i’ll bump those up.
Shouldn’t Ankara be accepted for one of those middle squares in Türkiye in beginner mode?
only European Turkish cities are in this quiz. Ankara considered part of Asia. It’s a Cyprus city in that square.
Ah of course, thank you
What is the city in west scotland in beginner?
Look closely, there is somewhere else in that square other than west scotland
Coleraine
At INT level I got all the squares cleared (269 of them) and the timer stopped. But the country counter still showed only 42/43 but with no countries remaining.
The same anonymous country fault appears on completion of the ADV level also: all 1118 squares completed and no countries remaining, but country counter still showing only 42/43 done. There’s something strage about Maribor also, as Slovenia showed as complete even though that sqaure was not entered.
There’s something amiss with the quiz now (at INT level). I’ts showing that there is only one country outstanding (RUS) but also showing that only 41/43 counries are complete. The city counter show 268 of 269 cities completed. There are two empty grid squares in Russia, but using the grid from the Europe City Latitude Coverage quiz for comparison (and various mapping sites), there is nothing in there above 10k to find. So how can the quiz be finished? (The empty grids are those south of Ukhta and east of Kandalaksha.)