Is "parece" only used for comparing visual similarities, or can it be used more in a general sense like the English word "seems"?
yes, it is also used to mean "to seem"
I am brazilian and seems is right
seems like a castle but it is a church
It's like a castle but it is a church.
It looks like a castle but it is a church.
I think "seems" is more correct than "It's like" ):
Don't we need to write "parece como um castelo"?
Nop...."parece um castelo"or "se parece com um castelo"
Appear ~~ seem. 'It appears a castle' should also be accepted
That sounds nonsensical in English; if you want to use "appears," you would have to add "to be" after it: "It appears to be a castle."
How about 'it resembles' a castle...?
«parecer-se com» = "to resemble;" «Se parece com um castelo...» = "It resembles a castle..."
How come you use the word "parece" for things that resemble each other? What is the etymology behind it?
It seems to be/It appears to be from late Latin, from the word for "appear:" http://www.priberam.pt/dlpo/parecer
Note that, in Italian, they use «sembrare», but there does exist a cognate «parere» that means the same thing.