Translation:This city is in the center of a large region.
The hint tells me that 'urbo' is 'city', not 'town'. Why? And how? To me, both 'city' and 'town' have always been urbo, while a 'village' is a vilaĝo. So what is 'town' then, if not an urbo?
Well, both "city" and "town" are usually called urbo, and every urbeto is automatically an urbo as well. Luckily, the contributors took out the hint (I think) and in any case, urbo is accepted for both "city" and "town" (and vice versa).
Because "great" means more than just "big"; it's more in the direction of "glorious, grand", which is more than is warranted by Esperanto "granda" which is simply "big, large".
I am aware of the connotation, however why not look at the simple denotation?! By the way in esperanto words also have a connotation and a denotation. Granda could mean "glorious / grand / famous / powerful / eminent / etc." but it could also mean "big/large". e.g. "Li estas granda arkitekto" would is most cases mean something like "He is a glorious / grand architect". However it could also mean "he is [physically] a big / large architect." 'Great' would cover all of the usages mentioned above (and more).
Yes, of course, why bother, Duolingo can't cover all possible translations, so sometimes translations are not accepted. I knew that before I ever used the app. It's just a computer program. Nevertheless, it sometimes leads to some minor frustrations and possibly an odd rhetorical question posted on a discussion page. (...)
(And by the way, I like some ambiguity! Isn't is 'great' that a word or phrase or maybe even a whole book can have more than one meaning!)