This is the 319th most frequent German word.
alt
"Alt" means "old" in English.
Here, 'alt' is used as an adjective to describe the noun 'book', referring to its age.
Das ist ein altes Buch.
That is an old book.
In this sentence, 'alt' is part of a phrase indicating one's age in years.
Wie alt bist du?
How old are you?
The word 'alt' functions as an adjective describing the castle, implying its historic or aged nature.