This is the 4574th most frequent Russian word.
городскую
"Городскую" means "urban" or "city-related" in English.
The word 'городскую' modifies 'архитектуру' (architecture) to describe it as pertaining to the city, using the feminine accusative singular form.
Я изучаю городскую архитектуру этого района.
I am studying the urban architecture of this region.
Here, 'городскую' describes 'площадь' (square), indicating it is city-related, also in the feminine accusative singular form.
Мы заблудились на городскую площадь.
We got lost at the city square.
In this sentence, 'городской' changes to the feminine prepositional case ('о городской') as it describes 'жизни' (life), expressing the concept of life in the city.