Ruins, when talking about the remains of what was once a building or city, is always used in the plural form. So you would say "I love these/those ruins."

You can say "the castle is a ruin". But if you were standing in the castle and admiring what was left of it, you would normally say "I love these ruins". You definitely can't say "this ruins" (unless you are using "ruins" as a verb, of course, e.g. "this always ruins my day"). This is because this/these has to agree with the noun, so if the noun is grammatically plural ("ruins") you have to use "these". "This ruin" is technically possible, but generally we use the plural.

I love these ruins :) But maybe even better would be something like, “these ruins are incredible”.