Learn Italian: Mi è piaciuta l'atmosfera in questa piazza. - I liked the atmosphere in this square.

In Italian, the verb piacere (to like) works a bit backwards for English speakers. The thing you like is actually the subject of the sentence doing the action. Because l'atmosfera (the atmosphere) is a feminine singular noun, the past participle has to match it, changing from piaciuto to piaciuta.

Why do we use è (is) instead of ho (I have) to say 'I liked'?

In the passato prossimo (past tense), piacere always uses essere (to be) as its helping verb, never avere. Instead of saying 'I have liked the atmosphere', Italians literally say 'The atmosphere is pleased to me' (mi è piaciuta).

If the subject is l'atmosfera, why doesn't the sentence start with it?

You can actually say L'atmosfera mi è piaciuta, but Italians often put the pronoun mi (to me) and the verb first when talking about things they like. This emphasizes who is experiencing the feeling.

Why does questa end in -a here?

Questa means 'this'. It ends in -a because it has to match the gender and number of the noun it describes, which is piazza (square) — a feminine singular word.