If the arcticle was needed, it would contract to nella (for feminine nouns)
in + la = nella = in the
I can't answer why the article is not required (it seems sometimes it is required, sometimes it is not), you could check out the examples on this page ... 'in città' is mentioned once on the site, and there are examples of other nouns that take in without the article
You may find this article helpful as far as when and when not to use the articolo determinativo: http://aulalingue.scuola.zanichelli.it/benvenuti/2009/11/26/uso-dellarticolo-determinativo/
To AdamKoziko: I think "in A city" means an indefinite city, any city as opposed to being in the country or in a rural area. When I say "THE city", it is the city I use to live in, it is MY city. Well, this is my opinion. I am not a native English speaker, though. I hope I have helped. Greetings. September 09.2016.
Thanks, alvaro1944. I am neither Italian nor English speaker, I'm Polish. In the Slavic languages we have no articles, that is why I still struggle with that. If there were here "Io sono nella città", I wouldn't have any questions. But there is no article here, so I've thought the "nul" article is closer to the indefinite article. Apparently I've been wrong.
To AdamKoziko: I think that now you understand the problem well. In the phrase "Io sono nella città" there is the article which corresponds to the English definite article 'the'. So the 'città' ('city') is the city the speaker lives in, id est, HIS city. In my native language, Portuguese, we have articles as well and the situation would be the same as it is in Italian. Greetings again. September 13, 2016.
From days of old, we have the phrase 'I am in town ...' in the UK, meaning I am available to meet up or carry out business face-to-face, especially in rural areas where market towns were the centre of commerce for farmers, etc. Possibly this could be the Italian equivalent.
you could omit the personal pronoun (io) if the subject is clear in the context (but it is not mandatory).
io / loro sono in città = I am / they are in the city
"sono" is the only conjugation that can have two subjects (I/they).
There is no other verb with this peculiarity