Learn French: Nous aimions ce quartier. En revanche, le loyer était cher. - We liked this neighborhood. On the other hand, the rent was expensive.

Both mean 'on the other hand' or 'however'. En revanche is slightly more formal and elegant than par contre. In everyday spoken French, you will hear par contre more often, but en revanche is great for writing or slightly more elevated speech.

Why are we using the imperfect tense (aimions, était) instead of the passé composé?

We use the imperfect here because we are describing past states or ongoing situations, not specific completed events. Liking the neighborhood and the rent being expensive were facts that were true over a period of time, setting the background.

Could I say cet quartier instead of ce quartier?

No, because quartier is a masculine word starting with a consonant, so it takes ce. You only use cet for masculine words that start with a vowel or a mute H (like cet hôtel or cet arbre) to make pronunciation smoother.

Is cher an adjective here? Does it change depending on the noun?