Learn Italian: Di notte la città è tranquilla.

Yes, you can say la notte to mean at night, but di notte is a very common set phrase meaning at nighttime or by night. Both can express a general truth or habit, but di notte is especially popular in Italian to describe the state of things during the night.

Why does the word città have an accent mark on the last letter?

The accent mark tells you to put the spoken stress on that final vowel. As a handy rule, Italian nouns that end in an accented vowel are invariable, meaning they do not change their ending in the plural. So, one city is la città and multiple cities would be le città.

Why is the word for quiet spelled tranquilla here instead of tranquillo?

In Italian, adjectives must match the gender and number of the noun they describe. Because la città is a feminine singular noun, the dictionary form of the adjective (tranquillo) changes its ending to match, becoming tranquilla.