Learn Spanish (Latin America): Ese vecindario era ruidosísimo.

Era is the imperfect tense of the irregular verb ser (to be). We use the imperfect tense to describe how things used to be or to set the scene in the past. Since the neighborhood's noisiness was an ongoing description without a specific start or end time, we use era rather than the preterite (completed past) form fue.

Could I use estaba instead of era here?

Generally, no. We use ser (and therefore era) to describe inherent, defining characteristics. By using era, you are saying that being noisy was a general trait of that neighborhood. If you wanted to say the neighborhood was noisy just at that specific moment (perhaps because of a loud parade passing through), you would use estar (estaba) to show it was a temporary condition.

What does the -ísimo ending on ruidosísimo mean?

It is a common way to say "very" or "extremely" in Spanish. Instead of using the word muy (very), you can take an adjective, drop its final vowel, and add -ísimo (or -ísima for feminine words). Here, the base word ruidoso (noisy) drops the -o and becomes ruidosísimo (extremely noisy).

Why do we use ese instead of eso for "that"?