The word "ruins" is more commonly used in the plural; however, the singular form, "ruin", can also be used in a wide range of different contexts.
The word "ruins" is most commonly used in the plural.
The Ancient ruins of Knossos are fascinating.
However, we use the singular form (ruin) in some contexts.
He faces financial ruin after losing the case.
When referring to the remains of a city or building, we frequently choose the plural form (ruins).
We explored the ruins of Delos last summer.
Be aware that you can use the plural form even if you're talking about a single site or building. In this situation, use a plural verb to say, for example, "ruins are" (not "ruins is").
The ruins of the building are overgrown with grass and shrubs.