Learn Spanish (Latin America): El museo está en la ciudad vieja.

We use está because we are talking about the location of a specific thing: el museo (the museum). If we wanted to say 'There is a museum' (unspecific), we would use hay instead: Hay un museo en la ciudad vieja.

Why does está have an accent mark on the 'a'?

The accent mark shows that the spoken stress goes on the last syllable. It also makes a big difference in meaning! está means 'is', but esta (no accent) means 'this', as in esta ciudad (this city).

How do we know ciudad is feminine if it doesn't end in 'a'?

Almost all Spanish nouns that end in -dad (like ciudad or universidad) are feminine. That is why it uses la and why the adjective that follows it is vieja.

Why is it ciudad vieja instead of vieja ciudad?

In Spanish, descriptive adjectives like vieja (old) almost always come after the noun they describe.