I understand that can is not a direct translation, but as an American speaker, I can tell you that "We see the castle from the roof" will actually be said as "We can see the castle from the roof". Part of translation and interpretation is transliteration of the meaning, not the actual words used. "How would a native express the same sentence" is to always be on the forefront of one's mind.
Not when beginning language instruction, using beginning software. There is a specific word for "can" or "to be able to", and that word, "potere" (in this case possiamo) was not offered here. Nor was riuscire. There are situations (odd ones, to be sure, but still plausible) where one might say "We see the castle from the roof".