This is the 17309th most frequent Polish word.
zamkniętego
"Zamkniętego" means "closed" (genitive or accusative singular masculine/neuter form) in English.
Here, 'zamkniętego' agrees with 'osiedla' (genitive singular form) to describe the gated aspect of the residential area.
Mieszkałem w domu zamkniętego osiedla.
I lived in a house within a gated residential area.
Here 'zamkniętego' describes 'pudełko' in a possessive context, indicating it is locked with a key, using genitive singular.
Pudełko zamkniętego na klucz nigdy nie otwieraliśmy.
We never opened the box that was locked with a key.
In this sentence, 'zamkniętego' describes 'sklepu' (genitive singular form), indicating the state of being closed.