The definite article is used a little differently in the two languages. In Italian it can be used to indicate a general category but in English no article is used to communicate the same thing. These sentences usually have no context, so it is often up to you whether to use the definite article or not :)
I disagree. The article is there to make it unambiguous that there are actually two or more real castes which can be seen from the house. "I see castles" is a generalized statement which may or may not target actual, existing castles. You could see castles in the clouds, castles in the mountains, castles in your mind's eye.
gli before plural masculine Words beginning with one of the following: a vowel, s + [consotant], gn, mn, pn, ps, x, and z. Sometimes before y as in lo Yoghurt.
I emphasize Words because the article can change, depending on word placement, particularly of adjectives, e.g.,, il pianoforte - lo stesso pianoforte.
Note that it's the sound that is determinative, so there are some rare nuances which don't warrant close attention now. Generally, in some words borrowed from foreign languages, the sound may make gli applicable: lo chef/gli chef (Observe no change to chef in plural form.)
For masculine singular words with these beginnings, use lo and uno, except before vowels, where lo becomes l' and uno becomes un (without an apostrophe!).
Irregular noun: Il dio ("the god") has the irregular plural gli dei