Learn Italian: Quel castello là è vecchio.

The word quello changes its shape depending on the next word, exactly like the definite article (the Italian word for "the"). Because you would say il castello (the castle), you use quel for "that". If the noun required lo instead of il (like lo specchio), you would use the full word quello (quello specchio).

What does là add to this sentence?

It adds emphasis, much like saying "that castle over there" in English. While quel castello already means "that castle", adding là (there) right after the noun really points out its physical distance from the speaker.

Why does là have an accent mark?

The accent mark changes the meaning. With the accent, là means "there". Without it, la means "the" (for feminine nouns) or "her". Italian often uses written accents on short words to tell apart two words that sound identical but mean different things.

Why does vecchio end in -o?

Vecchio means "old" and is an adjective describing the castle. Because castello is a masculine singular noun, the adjective's ending must match it perfectly, so it keeps its masculine singular -o ending.