It's "ruins are". Since "ruins" is a plural noun, it takes a plural form of a verb.

It's "ruins are". Since "ruins" is a plural noun, it takes a plural form of a verb.

These ancient ruins are the remains of an earlier civilization.

These ancient ruins is the remains of an earlier civilization.

Notice that we must use plural pronouns and determiners with plural nouns, like ruins, even if we are referring to the remains of a single building or city.

Those ruins are incredible. They can be found in Andalusia, Spain.

While "ruins" is most commonly used in the plural, the singular form can also be used to say, for example, "a ruin.".

The building is just a ruin with no roof.

Follow the same practice of using "are" with other related words that are commonly used in the plural, such as leftovers or remains.